Coating method

The coating method includes the intercoating step for spraying the intercoating paint over a substrate after undercoating, the intercoat drying step for drying the intercoat on the substrate, the overcoating step for spraying the overcoating paint over the dried intercoat on the substrate, and the overcoat drying step for drying the overcoat. In the intercoating step, the intercoating paint is sprayed in such an amount as causing sag, that is, in an amount beyond its sagging threshold value. In the intercoat drying step, the substrate is rotated about its horizontal and longitudinal axis to dry the intercoat.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a coating method. 
2. Description of Related Art 
Coating method of outer surfaces of substrates such as vehicle bodies 
generally includes steps of coating the substrates and drying coats 
thereon. The drying step may be divided further into a setting step and a 
baking step. The setting step is usually carried out in ambient atmosphere 
or at temperatures of 40.degree. to 60.degree. C. prior to the baking 
step. The temperature used for the setting step is lower than that for the 
baking step that is usually carried out at temperatures of approximately 
140.degree. C. The setting step may be referred to sometimes as temporary 
baking. 
In many cases coating on substrates may be effected at three steps: 
undercoating, intercoating and overcoating. Each coating comprises steps 
of providing a coat on a substrate and drying the coat thereon. An 
undercoat layer is usually formed on the substrate by means of dipping, 
while intercoat (intermediate coat) and overcoat (top coat) layers are 
generally formed by spraying. The paints used for each coating have their 
own functions: undercoating paints are used to ensure a resistance to 
corrosion, intercoating paints are to adjust a roughness of the undercoat 
and to provide an anti-chipping capability, and overcoating is to cover 
the intercoat. A total thickness of the three coat layers are generally in 
the range from 85 to 115 .mu.m: a thickness of the undercoat layer being 
usually in the range from 15 to 25 .mu.m; a thickness of the intercoat 
layer in the range from 35 to 45 .mu.m; and a thickness of the overcoat 
layer in the range from 35 to 45 .mu.m. 
Overcoating paints are extremely expensive compared with undercoating and 
intercoating paints so that it is desirable to allow a possibly thinner 
overcoating while an intercoat layer is rendered as thicker as possible in 
order to maintain a total thickness of all three coats to conventional 
thicknesses. 
As the overcoat would be made thinner, a color of the intercoat can be seen 
through the overcoat layer. This may be solved by using the intercoat 
having a color identical to or very similar to that of the overcoat layer, 
that is, by effecting the color intercoating. 
A limit exists, however, to the thickness of the intercoat layer because a 
thicker intercoat layer is likely to become roughened as if it is welted 
to a great extent. Such roughness on the intercoat layer offers the 
problem with evenness of an overcoat layer formed thereon. This roughness 
cannot be substantially restored by a wet rubbing treatment that is 
usually carried out after the drying of the intercoating. 
As one standard for evaluating the quality of a coat surface is a degree of 
evenness. The larger the degree of evenness becomes, the smaller 
irregularities or roughness on the coat surface. Accordingly, a coat 
having a larger degree of evenness may be determined as a better coat. It 
is known that such a degree of evenness on a coat may be improved as the 
coat is made thicker. 
As a paint is coated on the surface of a coating substrate, it may be 
caused to sag or drop downwardly along the substrate surface due to 
gravity. The paint may be likely to sag when a large amount of the paint 
is coated. This sagging phenomenon is a factor of adversely affecting a 
quality of the coated surface. 
The sag may be caused to occur due to the force of gravity so that it may 
occur on a vertical surface of the substrate. Accordingly, the sagging 
does not usually offer the problem when a large amount of a paint is 
coated on a transverse surface of the substrate. This results in the fact 
that a coat thickness of the transverse substrate surface can be rendered 
thicker than that of the vertical substrate surface. If a paint would be 
coated on the transverse surface of the substrate to form a coat as thick 
as a coat formed on the vertical surface thereof in such an amount as 
causing no sag on the transverse surface thereof, the paint coated on the 
transverse surface will be evened due to some extent of its natural flow 
whereby the coat having a higher degree of evenness is provided on the 
transverse surface than on the vertical surface. 
From the above point of view, conventional spraying procedures have taken 
the measures to prevent a paint from sagging by using a paint having a 
viscosity or flowability as low as possible in order to provide a coat 
surface with a high degree of evenness. For conventional overcoating 
paints such as thermosetting paints, a sagging threshold value is 
approximately 40 .mu.m at the maximum. The sagging threshold value is 
defined herein by the maximum thickness of a coated paint that cause no 
sags on the vertical surface of a substrate. Sagging of a paint is most 
likely to occur at the initial stages of the setting and baking steps and 
in particular at the initial stage of the baking step so that a thickness 
of a paint to be coated on a vertical substrate surface during the coating 
step is determined by the thickness of the coat that causes no sagging 
thereon at this stage, that is, by the sagging threshold value. In order 
to provide a coat layer with a higher degree of evenness, the conventional 
techniques require the paint to be coated plural times such as twice by 
repeating a series of the steps from the coating step to the baking step. 
For coating procedures requiring intercoating and overcoating steps, a 
degree of evenness of an intercoat layer exert a great experience on 
evenness or flatness on an overcoat layer to be coated thereon. As a limit 
exists in the conventional procedures, however, to improvement in a degree 
of evenness on the overcoat layer, a wet rubbing treatment has been 
carried out over the intercoat layer after the intercoat drying step in 
order to improve a degree of evenness on the intercoated layer. 
The use of the wet rubbing treatment necessarily require additional steps: 
the step for subjecting the dried intercoat to wet rubbing and the step 
for drying the wet intercoat after the wet rubbing step. An increase in 
these steps is disadvantageous from the point of view of commercial 
production. The wet rubbing may exercise an adverse influence upon a 
quality of a finish overcoat layer to be coated thereon because portions 
of a coating surface might be remained wet due to the difficulty of 
thoroughly drying such a vehicle body as having a complex construction 
with a variety and number of open portions. 
SUMMARY OF THE INVENTION 
The present invention has one object to provide a coating method adapted to 
cover an intercoat layer formed on an undercoat with an overcoat without 
subjecting the intercoat to a wet rubbing treatment or in such a manner as 
reducing the wet rubbing work. 
The present invention has another object to provide a coating method 
adapted to minimize a thickness of an overcoat, thereby reducing an amount 
of an expensive overcoat paint. 
The present invention has a further object to provide a coating method 
adapted to produce an overcoat with a higher degree of evenness compared 
with another overcoat with the same film thickness. 
In one aspect the present invention consists in a coating method which 
comprises the first step of spraying an intercoating paint over a surface 
of a substrate to form an intercoat layer thereon on which an undercoat 
has been layered, an amount of said intercoating paint to be sprayed at 
least on a vertical surface of the substrate being larger than such an 
amount thereof as causing said intercoating paint to sag; the second step 
of drying said intercoat layer while the substrate is being rotated about 
the horizontal axis therof; the third step of spraying an overcoating 
paint over said intercoat layer to form an overcoat layer after the second 
step; and the fourth step of drying said overcoat layer. 
The coating method according to the present invention is designed such that 
the substrate on which the intercoating paint has been sprayed is rotated 
about the horizontal and longitudinal axis thereof for drying the 
intercoated paint whereby the force of gravity acting in the vertical 
direction on the intercoat formed on the vertical surface of the substrate 
is forced to be altered. The alteration of the direction of gravity 
prevents the intercoated paint from sagging during the drying step and the 
paint is dried without sagging even if the paint has been sprayed in an 
amount large enough to cause sagging. Thus the features of the coating 
method according to the present invention involve the spraying of the 
intercoating paint in an amount larger than a sagging threshold value, 
that is, in such an amount as causing the paint to sag, and the rotating 
the substrate on which the intercoating paint has been coated in the 
horizontal axis thereof. These features of the present invention provide 
an overcoat with a higher degree of evenness than an overcoat produced by 
conventional techniques if their intercoats were as thick as each other 
and at the same time permits an intercoat much thicker than conventional 
ones can. 
Even if no wet rubbing is carried out, the coating method according to the 
present invention provides a dried intercoat layer with a degree of 
evenness higher than or as high as such a dried intercoat layer as have 
been produced by subjecting the intercoat layer to the wet rubbing 
treatment. Without the wet rubbing, this results in provision of a final 
overcoat with a degree of evenness higher than or as high as overcoats 
formed by conventional procedures requiring the wet rubbing treatment for 
the intercoat. Even if the wet rubbing is preferably employed, a workload 
for the wet rubbing treatment for providing a desired degree of evenness 
can be reduced to an extent less than that being otherwise required for 
the wet rubbing in the conventional procedures. 
Furthermore, in instances where it is not necessary to provide a final 
overcoat surface with a very high degree of evenness, it is found 
advisable that an intercoat can be made as thick as possible and a degree 
of evenness on the intercoat surface is rendered as high as possible. This 
can help decrease an amount of the overcoated layer and reduce an amount 
of an overcoating paint to be used whereby painting costs are to be 
reduced because the overcoating paint is much expensive compared with the 
intercoating paint. If the intercoating paint having a color identical or 
extremely similar to the color of the overcoating paint is employed as in 
a so-called color intercoating, a thickness of the overcoat layer can be 
rendered extremely thin. 
It is also to be noted that the coating method according to the present 
invention permits an overcoating paint to be coated by spraying the paint 
at least on the vertical surface of the vehicle body W to a thickness that 
the paint is caused to sag. In this case, the paint sprayed on the dried 
intercoat on the substrate is dried while being rotated about the 
horizontal axis thereof in the overcoat drying step. This procedure 
provides an overcoat surface with a higher degree of evenness compared 
with overcoats with identical thicknesses prepared by conventional 
procedures. In other words, the present invention can provide an overcoat 
with a high degree of evenness even if a thickness of the overcoat is made 
thinner. It is necessarily possible to improve a degree of evenness on an 
overcoat surface to a remarkably high extent if the overcoat is rendered 
thick. 
In accordance with the present invention, it is found preferable to change 
a conveyer or carrier means, such as carriages, for conveying the 
substrate from one step to another. In other words, it is preferred that 
the substrate is transferred from a carriege on which it is loaded during 
the intercoating step to another carriage on which it is to be loaded 
during the intercoat drying step. The change of the carrier means can 
prevent dirts or other foreign materials from adhering onto the wet 
intercoat surface during the intercoat drying step because the intercoated 
substrate is rotated during the intercoating drying step while it is 
loaded on the carrier means. In particular, in instances where a paint 
adheres to a rotation mechanism mounted on the carrier means for rotating 
the substrate, it shows a growing tendency that it comes off or peels off 
as it become solidified particularly on rotating and sliding portions of 
the rotation mechanism of the carrier means. Scales or particles of the 
solidified paint peeled off from the carrier means suspend in air in the 
form of floating dust and they are likely to adhere to the wet intercoat 
surface of the substrate. This possibility can be prevented or reduced to 
a minimized level by transferring the intercoated substrate to a new 
carriage that has not been employed for spraying the substrate with the 
intercoating paint. 
The change of the conveyance or carrier means is preferably made like the 
intercoat drying step in instances where the substrate with the overcoat 
is rotated during the overcoat drying step. 
In order to prevent dust and other foreign materials from adhering to the 
coated surface, the substrate may be preferably rotated during a 
preparation step prior to the step of spraying the substrate with a paint 
to remove the dust and so on therefrom. The rotation of the substrate 
permits a sufficient removal of the dust and so on by causing them to fall 
down therefrom as the substrate is being made a turn. 
It is furthermore preferred that the substrate is subjected to correction 
coating prior to the spraying of the intercoating paint. Although the 
substrate is sprayed with the intercoating paint by means of automatically 
spraying machines such as robots, there are some portions that remain 
irregular in spraying or incompletely painted. In conventional procedures, 
such irregularly and incompletely sprayed portions are subjected to 
correction coating by means of manual spraying. For the coating method 
according to the present invention, at least the intercoating paint is 
sprayed in an amount larger than a sagging threshold value at the 
intercoat spraying step so that the substrate should be conveyed to the 
intercoat drying step as soon as possible in order to prevent the paint 
from falling down therefrom. The provision of the correction coating in 
advance prior to the intercoat spraying step permits a quick transfer of 
the substrate to the intercoat drying step. Such portions as 
irregularities or incompleteness in the spraying of the intercoating paint 
may be experimentally determined in advance. This can be also said true of 
the overcoat spraying step in which the overcoating paint is sprayed in 
such an amount as exceeding its sagging threshold value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Outline of the Coating Method 
FIG. 1 shows an outline of the steps of the coating method according to the 
present invention, in which a vehicle body W as a substrate is coated from 
step P1 to P13. 
To a preparation step P1 is conveyed the vehicle body W by a carriage D 
after an undercoating paint was coated by means of conventional 
electrodeposition in known manner. In the preparation step P1, dust and 
other foreign materials are removed from the vehicle body W, for example, 
by vacuum suction. 
In step P2, the vehicle body W is transferred from the carriage D on which 
it is loaded during the preparation step P1 to a step P3 in which an 
intercoating paint is sprayed. In step P4, the vehicle body W is then 
transferred to another carriage D, and the intercoat is dried in a setting 
step P5 and a baking step P6. 
After the drying, the vehicle body W is transferred to another carriage D 
in a step P7 and conveyed to steps P8 to P13 for the overcoating. The 
steps P8 to P13 are substantially the same as the steps P1 to P6 except 
that an overcoating paint is used in place of the intercoating paint. A 
description of the steps P8 to P13 will be omitted for avoidance of 
explanation in duplicate. The vehicle body W is then conveyed after 
completion of the overcoating to an assembly line. 
Spraying and Drying of Paints 
Paints are sprayed in the step P3 for the intercoating and in the step P10 
for the overcoating, and the coated paints are dried in the steps P5 and 
P6 for the intercoating and in the steps P12 and P13 for the overcoating. 
As have mentioned immediately hereinabove, the steps P3 to P6 are 
substantially the same as the steps P10 to P13 so that only the steps P3 
to P6 for the intercoating will be described in detail and a description 
on the steps P10 to P13 will be omitted for brevity of explanation. 
The intercoating paint is sprayed in the step P3 on the undercoat surface 
of a substrate such as the vehicle body W in an amount larger than a 
sagging threshold value. The paint usually used for intercoating purposes 
has a sagging threshold value of approximately 40 .mu.m; in this step P3, 
the intercoating paint is sprayed in an amount much larger than the 
sagging threshold value, for example, 60 .mu.m. An estimated correction 
coating is carried out prior to this intercoating. This will be described 
in detail as well as the spraying of the paint in an amount beyond the 
sagging threshold value. 
After the intercoating in the step P3, the vehicle body W is transferred to 
the different carriage D in the step P4 as quick as possible and it is 
conveyed to the step P5 where the coated paint is set. In the setting step 
P5, the vehicle body W is turned about its axis l extending horizontally 
or longitudinally (in this example) through the vehicle body W as shown in 
FIGS. 2(a) to 2(i). A temperature profile for the setting step P5 may be 
ambient or elevated in a range from 40.degree. to 60.degree. C., and the 
temperature profile for the setting step P5 is lower than a temperature 
profile for the baking step P6. The setting step P5 serves as evaporating 
paint components having low boiling points in advance to avoid a rapid 
evaporation thereof in the baking step P6 whereby formation of pinholes 
can be avoided. 
In the baking step P6, temperatures in the range higher than that applied 
to the setting step P5 are added to the vehicle body W and the intercoated 
layer is baked. The vehicle body W is rotated in this step in 
substantially the same manner as in the setting step P5 as shown in FIGS. 
2(a) to 2(i). 
The rotation of the vehicle body W about its horizontal axis in the steps 
P5 and P6 permits the drying of the paint coated in an amount larger than 
its sagging threshold value without sagging whereby there is provided an 
intercoat with such a higher degree of evenness that conventional 
procedures cannot provide. It is thus possible to avoid the use of a wet 
rubbing treatment which has been conventionally used between the intercoat 
spraying step P3 and the intercoat setting step P5. 
Changes of the carriages D in the steps P4 and P11 are made in order to 
avoid adhesion of the paints to the carriages D to be used in the drying 
steps P5, P6, P12 and P13. In the spraying steps P3 and P10, sprays of the 
paints are caused to adhere to the carriages D with the vehicle body W 
loaded thereon so that such sprays may be caused to come off from the 
carriages D and consequently suspended as dust in air if the carriages D 
are used to convey the coated vehicle body W to the following drying 
steps. Such dust may adhere to the vehicle body W leading to an impairment 
of the quality of the coat surface. Changes of the carriages D in the 
steps P2, P7 and P9 are also made in order to minimize adverse influences 
from floating dust in air upon the coat surface; however, the necessity of 
these steps is not so important as the changes of the carriages D in the 
steps P4 and P11. 
Relationship of Evenness with Rotation and Wet Rubbing 
FIG. 3 shows a graph demonstrating the relationship of degrees of evenness 
with film thicknesses of overcoats when intercoats with varying film 
thicknesses are formed. This graph is shown to demonstrate influences of 
the degrees of evenness on the intercoat surfaces upon degrees of evenness 
on the overcoat surfaces. The degree of evenness on the coat surface may 
be represented by a known PGD value which represents a degree of 
identification of an image reflected from the coat surface. It is 
understood that the degree of evenness becomes higher as the PGD value 
gets larger. As shown in FIG. 3, it is noted that, as the PGD values of 
the intercoat surfaces increase from 0.2 through 0.4 and 0.6 to 0.8, the 
degrees of evenness on the corresponding overcoat surfaces get higher. It 
is further noted that, in instances where the degrees of evenness on the 
intercoat surfaces are identical to each other, the overcoat surface with 
a thicker film thickness provides a higher degree of evenness. 
The data shown in FIG. 3 were obtained under the following test conditions: 
For intercoating: 
Paint: Polyester melamine 
Color: Gray 
Viscosity: 22 seconds/Ford Cup #4 
Film Coater: 
Minibell 
Sprayed twice at the interval of 3 minutes 
For overcoating: 
Paint: Polyester melamine 
Color: Red 
Viscosity: 20 seconds/Ford Cup #4 
Film Coater: 
Minibell 
Sprayed twice at the interval of 3 minutes 
FIG. 4 shows a graph demonstrating relationships of degrees of evenness on 
intercoat surfaces with film thicknesses of intercoats. This graph 
indicates an influence of the rotation of the intercoated substrates upon 
the degrees of evenness on the intercoat surfaces. As shown by the broken 
lines in FIG. 4, it is to be understood that, although the degrees of 
evenness on the intercoat surfaces produced by conventional procedures can 
be improved as the intercoats are made thicker, they are too low so that 
the wet rubbing treatment is required to raise their degrees of evenness 
to a greater extent. On the contrary, it is understood from the graph 
shown by the solid line in FIG. 4 that, in instances where the 
intercoating paints are sprayed in amounts beyond their sagging threshold 
values and the intercoats have been dried while the intercoated substrates 
have been turned, the degrees of evenness on the intercoat surfaces have 
been improved to a remarkable extent compared to those obtained by 
conventional techniques. 
The data shown in FIG. 4 were obtained by using the same intercoating paint 
as used for the tests as shown in FIG. 3 under the following test 
conditions: 
For wet rubbing: 
Using a water-resistant paper #800, the test intercoat surface is 
wet-rubbed uniformly until its gloss is caused to disappear. 
For rotation of the substrate: 
The substrate is rotated for the full period of 10 minutes for the setting 
step P5 and for the initial period of 10 minutes for the baking step P6 
while a speed of rotation of the substrate is 10 r.p.m. over the drying 
period. 
In both cases, the sagging threshold values of the coating paints were 
changed by changing concentrations of a thinner. 
Relationship of Film Thickness with Sagging Threshold Values and Rotation 
of Substrates with Degrees of Evenness 
FIG. 31 demonstrates influences of film thicknesses upon sagging threshold 
values using three different film thicknesses of 40 .mu.m, 53 .mu.m, and 
65 .mu.m. It is understood from FIG. 31 that in each case a peak of the 
sag has occurred at the initial stages of the setting and baking steps. 
The sagging threshold value is usually defined as a value at the time when 
the sag is caused to occur at a rate ranging from 1 to 2 mm per minute. It 
is understood that, if the sag would occur at a rate of 2 mm or more per 
minute when visually observed, the coat surface is made not good. The 
maximum film thicknesses of conventional paints that had ever obtained at 
a range below the sagging threshold value were as thin as about 40 .mu.m. 
FIG. 32 shows influences of horizontal rotations of the vehicle body W 
about the horizontal and longitudinal axis upon degrees of evenness on 
overcoat surfaces. In FIG. 32, reference symbol A denotes a state of the 
overcoat obtained using a conventional coating method where the vehicle 
body W is not subjected to rotation. Reference symbol B denotes a state of 
the overcoat or top coat obtained by rotating the vehicle body W in the 
clockwise direction at 90.degree. and then reversing it in the 
counterclockwise direction to the original position, namely, rotating it 
from the position of FIG. 2(a) through (b) to (c) and then reversing it 
from the position (c) through (b) back to (a). Reference symbol C denotes 
a state of the top coat obtained by rotating the vehicle body W at 
135.degree. and then reversing it to the original position, namely, 
rotating it from the position of FIG. 2(a) through (b) and (c) to (d) and 
then returning it from the position of FIG. 2(d) through (c ) and (b) back 
to the original position (a). Reference symbol D denotes a state of the 
top coat obtained by rotating the vehicle body W at 180.degree. from the 
position of FIG. 2(a) through (b), (c) and (d) to (e) and then back to the 
original position of FIG. 2(a) through (d), (c) and (b) from (e). In FIG. 
32, reference symbol E denotes a state of the overcoat obtained when the 
vehicle body W is rotated around in one way from the original position of 
FIG. 2(a) through (b), (c), (d), (e), (f), (g) and (h) back again to the 
original position of FIG. 2(i). 
As is apparent from the results of FIG. 32, it is to be understood that the 
top coats with higher degrees of evenness are gained when the vehicle body 
W is rotated, as shown by the reference symbols B, C, D and E in FIG. 32, 
than when it is not rotated, as shown by the reference symbol A in FIG. 
32, in instances where film thicknesses of the top coats are identical to 
each other. It is noted that, in instances where the vehicle body W is 
rotated, the round rotation of the vehicle body W in one direction by 
360.degree. is preferred to provide a top coat with a higher degree of 
evenness. It is also noted that, in instances where the vehicle body W is 
not rotated as in conventional coating methods, a limit exists to the film 
thickness of the top coat and thus to the degree of evenness. 
As shown in FIG. 32, in instances when the film thickness of 65 .mu.m was 
formed on the vehicle body W by rotating it by 360.degree. in one 
direction, an image sharpness degree I.G. of the overcoat was found to be 
87, namely, the lowest limit value when the PGD value is 1.0. When the 
film thickness of 40 .mu.m was formed without rotation of the vehicle body 
W, an image sharpness degree I.G. was found to be 58 (the lowest limit 
value when the PGD value is 0.7), while an image sharpness degree I.G. was 
found to be 68 (the lowest limit value when the PGD degree is 0.8) when 
the vehicle body W is rotated at 360.degree.. 
In the above definition terms, an image sharpness degree I.G. (image gloss) 
is a percentage of an objective image sharpness when it is defined as 100 
when a mirror surface of a black glass is used, and a PGD value is a value 
rating identification degrees of reflected images from 1.0. The values get 
lower as degrees of evenness get lower. 
The data shown in FIGS. 31 and 32 were obtained under the following test 
conditions and these conditions are the same as those used for the steps 
P10, P12 and P13: 
(a) Paint: melamine alkyd (black) 
Viscosity: 22 seconds/20.degree. C. (measured by Ford Cup #4) 
(b) Film coater: 
Minibell (16,000 r.p.m.) 
Shaping air: 2.0 kg/cm.sup.2 
(c) Spraying amounts: sprayed two times 
First time: 100 cc/minute 
Second time: 150-200 cc/minute 
(d) Setting time/temperature: 10 minutes/room temperature 
(e) Baking temperature/time: 140.degree. C./25 minutes 
(f) Degree of undercoat evenness: 0.6 (PGD value) (intercoat over PE tape) 
(g) Time period for rotation and reversal: 
10 minutes (for the setting step) 
10 minutes (for the baking step) 
(h) Material to be coated: The side surfaces of a square pipe with a 30 cm 
side are coated and supported at its center rotatively. 
(i) Rotational speed of the material to be coated: 6, 30 and 60 r.p.m. (It 
is noted that no difference has in fact been recognized therebetween.) 
Color Intercoating 
In order to minimize the film thickness of the overcoat or top coat, there 
is used in the step P3 the intercoating paint having a color identical or 
similar to a color of the overcoat to be used in the overcoating step P10. 
In this case, the film thickness of the intermediate coat or intercoat to 
be sprayed in the step P3 is rendered larger while the film thickness of 
the overcoat to be sprayed in the step P10 is rendered smaller, for 
example, 15 to 30 .mu.m. And the overcoating paint is sprayed in this case 
in such an amount as exceeding its sagging threshold value, and the 
substrate such as the vehicle body W on which the paint has been 
overcoated is turned in the steps P11 and P12. 
The following table shows effects of the color intercoating on the 
overcoats formed on the intercoat. 
TABLE 
__________________________________________________________________________ 
Degree of Evenness 
Film Thick- 
Film Thick- 
Film Thick- Anti- 
(PGD value) 
ness of 
ness of 
ness of 
Masking 
chip- 
I 
Undercoat 
Intercoat 
Overcoat 
Ability 
ping 
A B II 
__________________________________________________________________________ 
20 50 25 .circle. 
3 0.6 
0.8 1.2 
20 50 30 .circle. 
4 0.7 
0.9 1.2 
20 60 15 .circle. 
5 0.5 
0.7 0.8 
20 60 20 .circle. 
5 0.6 
0.8 1.2 
20 70 15 .circle. 
5 0.6 
0.8 0.9 
20 70 20 .circle. 
5 0.7 
0.9 1.2 
__________________________________________________________________________ 
In the above table, the film thickness of the coat means an average film 
thickness of all the measured thinner and thicker portions of the coat and 
the symbol and numerals denote the following: 
For masking ability: 
The reference symbol "O" denote that the masking ability is so sufficient 
that no color of the intercoat can be seen through the overcoat. 
For anti-chipping: 
Determination has been done by the number of rust spots using Gravello 
test. 
5:0-5 rust spots (excellent) 
4:5-15 rust spots (good) 
3:16-30 rust spots (average) 
2:31-50 rust spots (poor) 
1: more than 50 rust spots (bad) 
Details of the Gravello test are as follows: 
1. Gravello tester and materials therefor: 
nozzle size: 50% 
distance: 300 mm 
stone: JIS #A500/7 crushed stone, 30 grams 
air pressure: 2.5 kg/cm.sup.2 
II. Method: After the test, salty water was sprayed for 72 hours and the 
number of rust spots occurred was counted. 
For degree of evenness (PGD value): 
I: The overcoat was formed by conventional coating procedures. Reference 
symbol "A" denotes that no wet rubbing was done after the intercoat had 
been dried, and "B" denotes that the intercoat was wet rubbed after dried. 
II: The overcoat was formed in accordance with the coating method according 
to the present invention, in which the overcoat was sprayed in the amount 
beyond its sagging threshold value and the substrate was rotated during 
the drying step while no wet rubbing was effected. 
Details of the coating are as follows: 
For substrate: 
A steel plate treated with zinc phosphate was used as a substrate. 
For undercoating: 
cationic electrodeposition (black) 
thickness: 20 .mu.m 
baking: 170.degree. C., 30 minutes 
For intercoating: 
paint: polyester melamine 
color: white (to comply with the color of the overcoat) 
viscosity: 24 seconds/Ford Cup #4 
film coater: Minibell, sprayed twice at the interval of 3 minutes 
baking: 140.degree. C., 25 minutes 
For overcoating: 
paint: polyester melamine 
color: white 
viscosity: 
a. For evenness degree classification "I" in the above table: 
Among paints having the viscosity of 16-22 seconds/Ford Cup #4, the paint 
was chosen which has a possible small viscosity in the range in which no 
sags occur. 
b. For evenness degree classification "II" in the above table: 
For film thicknesses of 25 .mu.m or more, 16 seconds/Ford Cup #4 
For film thicknesses of 20 .mu.m or less, 13 seconds/Ford Cup #4 
film coater: Minibell, sprayed once 
baking: 140.degree. C., 25 minutes 
For rotation of the substrate: 
10 r.p.m. for 10 minutes at the initial stage of the 
setting step and another 10 r.p.m. for additional 10 minutes at the initial 
stage of the baking step 
As will be apparent from the above table, the coating method according to 
the present invention can provide the overcoat or top coat with excellent 
anti-chipping and masking abilities and sufficiently high degrees of 
evenness. It further permits spraying of the overcoating paint to a 
thinner film thickness while rendering the intercoat thicker. 
From the data shown in the above table, it is to be noted that the coating 
method according to the present invention provides overcoats having 
extremely higher degrees of evenness represented by the PGD values usually 
over 1.0 without wet rubbing. In the conventional coating method in which 
the intercoat and overcoat layers are formed by spraying in each case to 
the film thickness of 40 .mu.m, the overcoat surface can give the PGD 
value of approximately 0.7 even if the intercoat was subjected to wet 
rubbing after it was dried. 
It is further to be noted that a combined film thickness of the intercoat 
and overcoat layers may be in the range preferably from approximately 70 
to 100 .mu.m from the point of view of ensuring a sufficiently thick coat 
as in conventional coating methods. In this preferred range, the film 
thickness of the intercoat layer may be larger than approximately 50 .mu.m 
that is thicker than conventional intercoat layers and it may be 
preferably larger than approximately 60 .mu.m in order to ensure better 
anti-chipping ability. Furthermore, the film thickness of the overcoat may 
be thinner than approximately 30 .mu.m that is thinner than conventional 
intercoats and it is further preferably in the range as thin as from 
approximately 20 to 25 .mu.m in order to adequately reduce a consumption 
of the overcoating paints which is much more expensive than the 
intercoating paints. It is also possible to make the overcoat a film 
thickness of 15 .mu.m; however, this thin overcoat will give a less degree 
of evenness than a thicker overcoat. 
In instances where a paint is sprayed to a thinner film thickness but in 
such an extent as causing sags, the paint may be conveniently prepared by 
adjusting amounts or ratios of resin components of the paint and solute 
components thereof. 
The coating method according to the present invention will be described by 
reference to a coating system and aparatus adapted to be designed 
therefor. 
Rotation Jig 
An example of a rotation jig mounted on a carriage D will be described in 
detail which is used for supporting the substrate such as the vehicle body 
W and rotating the substrate about its horizontal and longitudinal axis. 
Referring to FIGS. 5 and 6, the rotation jig is shown to include a front 
jig portion 1F mounted to the front side of the vehicle body W and a rear 
jig portion 1R mounted to the rear side thereof. 
As shown in FIG. 5, the front jig portion 1F includes a pair of left and 
right mounting brackets 2, 2, a pair of left and right stays 3, 3 welded 
to the corresponding mounting brackets 2 and a connection bar 4 connecting 
the pair of the stays 3, 3, and a rotary shaft 5 connected integrally to 
the connection bar 4. The front rotation jig 1F is fixed through the 
brackets 2, 2 to the forward end portions of a front reinforcing member of 
the vehicle body W such as front side frames 11, 11. The front side frames 
11, 11 are usually provided with brackets 12, 12 for mounting a bumper 
(not shown) so that the brackets 2, 2 are fixed detachably with bolts (not 
shown) to the brackets 12, 12. 
Referring now to FIG. 6, the rear rotation jig 1R is shown to have 
substantially the same construction as the front rotation jig 1F. In FIG. 
6, the elements of the rear rotation jig 1R having the same function are 
provided with the same reference numerals as the front rotation jig 1F and 
a new description on those elements will be omitted here for brevity of 
explanation. The rear rotation jig 1R is mounted to the vehicle body W by 
fixing the brackets 2, 2 with bolts to floor frames 13, 13 disposed at the 
rear end portion of the vehicle body W as a rigidity adding member. As the 
rear end portion of the floor frames 13 are usually welded in advance with 
brackets for mounting bumpers, the rear rotation jig 1R may be mounted to 
the brackets for mounting the bumpers. 
The front and rear rotation jigs 1F and 1R are mounted in such a state that 
their respective rotary shafts 5 are disposed so as to allow their common 
rotation axis l to coincide with each other and be in a straight line 
extending in the longitudinal direction of the vehicle body W. It is 
preferred that the rotation axis l is designed to pass through the center 
of gravity G of the vehicle body W as shown in FIG. 7. This arrangement 
for the rotation axis serves as preventing a speed of rotation from 
deviating to a large extent, thereby diminishing shocks originating from a 
deviation of rotations. Such shocks may cause a disorder in sagging so 
that this arrangement of mounting the front and rear rotation jigs 1F and 
1R is advantageous in prevention of undesirable sags from occurring. 
The front and rear rotation jigs 1F and 1R may be prepared for exclusive 
uses according to kinds of vehicle bodies. 
Carriages 
The carriages are used for transferring the vehicle body W in the steps P5, 
P6, P12, and P13. Each of the carriage D used therein is provided with a 
mechanism for rotating or turning the vehicle body W loaded thereon. 
Referring to FIG. 5, the carriage D is shown to include a base 21 and 
wheels 22 mounted to the base 21 with the wheels 22 arranged to 
operatively run on rails 23. On the base 21 is mounted one front support 
24, two intermediate supports 25 and 26, and one rear support 27, each 
standing upright from the base 21, as shown in the order from the right to 
left in FIG. 7. Between the intermediate supports 25, 26 and the rear 
support 27 is formed a supporting space 28 extending in a widely spaced 
relationship in the longitudinal direction. 
The vehicle body W is loaded on the carriage D and supported in the 
supporting space 28 in such a manner that the front end portion of the 
vehicle body W is rotatively supported through the front rotation jig 1F 
to the intermediate support 26 while the rear end portion thereof is 
rotatively supported likewise through the rear rotation jig 1R to the rear 
support 27. 
The front and rear rotary shafts 5 of the respective front and rear 
rotation jigs 1F and 1R are connected to the intermediate support 26 and 
the rear support 27 so as to be detachable from the vertical direction. 
The rear rotary shaft 5 of the rear rotation jig 1R is engaged with the 
rear support 27 so as to be not movable in the direction of the rotation 
axis l. At this end, the intermediate support 26 is provided at its top 
end surface with a cut-out portion 26a opening upwardly as shown in FIGS. 
10, 11 and 12, while the rear support 27 is provided at its top end 
surface with a cut-out portion 27a opening upwardly as shown in FIGS. 10, 
14 and 15. These cut-out portions 26a and 27a are formed in a size large 
enough to allow the front and rear rotary shafts 5 to fit around them, 
respectively, and be inserted thereinto. The rear rotary shaft 5 of the 
rear rotation jig 1R is provided with a flange portion 5a, and the rear 
support 27 is provided with a second cut-out portion 27b in a shape 
corresponding to and engageable with the flange portion 5a of the rear 
rotary shaft 5R communicating with the first cut-out portion 27a. This 
construction permits the connection or disconnection of the rear rotation 
jig 1R to or from the first and second cut-out portions 27a and 27b of the 
rear support 227 in a downward or upward direction. This construction also 
permits the rear rotation jig 1R to be securely connected to the rear 
support 27 by means of the stopper action by the flange portion 5a so as 
to be not movable in either of the forward and backward directions. A 
force of rotation for turning the vehicle body W loaded on the carriage D 
is applied to the vehicle body W through the front rotary shaft 5 of the 
front rotation jig 1F. At this end, the front rotary shaft 5 thereof is 
provided at its forward end portion with a connection portion 5b (see also 
FIG. 5) as will be described later. 
From the base 21 extends downwardly a stay 29 to a lower end of which is 
connected a retraction wire 30 that is of endless type and is driven in 
one direction by a motor (not shown). The retraction wire 30 thus drives 
the carriage D in a predeterminated conveyance direction. The motor should 
be disposed in an explosion proof place. 
A rotation of the vehicle body W is carried out using a movement of the 
carriage D, that is, using a displacement of the carriage D with respect 
to the rails 23. The displacement of the carriage D is converted into a 
force of rotation by means of a converting mechanism 31 which includes a 
rotary shaft 32 supported rotatively by the base 21 and extending 
vertically from the base, a sprocket 33 fixed on the lower end portion of 
the rotary shaft 32, and a chain 34 engaged with the sprocket 33. The 
chain 34 is disposed parallel to the retraction wire 25 and in such a 
state that it does not move along the rails 23. With this arrangement, as 
the carriage D is retracted by the retraction wire 25, the sprocket 33 
engaged with the unmovably mounted chain 34 allows the rotary shaft 32 to 
rotate, thus leading to the rotation of the vehicle body W. 
The rotation of the rotary shaft 32 is transmitted to the front rotary 
shaft 5 of the front rotation jig 1F through a transmitting mechanism 35. 
The transmitting mechanism 35 includes a casing 36 fixed on the rear side 
surface of the front support 24, a rotary shaft 37 supported rotatively to 
the casing 36 and extending in the transverse direction, a pair of bevel 
gears 38 and 39 for rotating the rotary shaft 37 in association with the 
rotary shaft 32, and a connection shaft 40 connected to the front support 
25 rotatively and slidably in the longitudinal direction. The connection 
shaft 40 is connected to the rotary shaft 37 by means of the spline 
connection system at a position represented by 41 in FIG. 7. This 
construction permits a rotation of the connection shaft 40 in association 
with the rotation of the rotary shaft 32. The rotary shaft 37 and the 
connection shaft 40 are arranged so as to allow their rotation axis l to 
coincide with each other in the longitudinal direction. 
The connection shaft 40 is connected to or disconnected from the front 
rotary shaft 5 of the front rotation jig 1F. As shown in FIGS. 10 to 12, 
the front rotary shaft 5 of the front rotation jig is provided at its tip 
portion with a connecting portion 5b in a cross shape, while the 
connection shaft 40 is provided at its rear end portion with a box member 
40a having an engaging hollow portion 40c engageable tightly with the 
connecting portion 5b of the front rotary shaft 5 as shown in FIGS. 10 and 
13. By moving the connection shaft 40 in a sliding manner through a rod 
43, for example, using a hydraulic cylinder 42, the connecting portion 5b 
is allowed to be connected to or disconnected from the engaging hollow 
portion 40c of the box member 40a. At the time of connection, the 
connection shaft 40 is rotatable integrally with the rotary shaft 5. The 
rod 43 is disposed in a ring groove 40b formed on the outer periphery of 
the box member 40a, as shown in FIG. 10, in order to interfere with the 
rotation of the connection shaft 40. 
With the above arrangement, the front and rear rotary shafts 5, 5 of the 
respective front and rear rotation jigs 1F and 1R are allowed to be 
supported to the intermediate support 26 and the rear support 27 in such a 
manner as being rotatable but unmovable in the forward and rearward 
directions, when the vehicle body W is lowered down to be loaded on the 
carriage D in a state of the connection shaft 40 being displaced toward 
the right in FIG. 7. Thereafter the connecting portion 5b of the rotary 
shaft 5 is engaged with the engaging hollow portion 240c of the connection 
shaft 40, whereby the vehicle body W is allowed to rotate about the 
predetermined rotation axis l by retracting the carriage D by means of the 
retraction wire 30. 
The vehicle body W can be unloaded from the carriage D in the reverse 
order. 
Referring now to FIGS. 16 to 19, there are shown examples of variants in 
connection systems between the rotation jig 1F and the connection shaft 
40, in which the same elements are represented by the same reference 
numerals. 
As shown in FIGS. 16 and 17, a cut-out portion 26a of the intermediate 
support 26 is formed in such a semi-circular shape as capable of 
rotatively supporting a box member 40a. And a connecting portion 5b-1 of 
the front rotary shaft 5 of the front rotation jig 1F is formed in an 
L-shaped manner, while an engaging portion 40c-1 of the box member 40a is 
formed in such a shape that the L-shaped connecting portion 5b-1 is 
engaged unrotatively relative to the engaging portion 40c-1 thereof. The 
engaging portion 40c-1 has an opening on one side surface of the box 
member 40a. As the opening is directed upwardly, the front rotary shaft 5 
of the front rotation jig is connected at its connecting portion 5b-1 to 
or disconnected from the connection shaft 40 through the engaging portion 
40c-1. 
FIGS. 18 and 19 show another example of a connection arrangement similar to 
that shown in FIGS. 16 and 17. The connecting portion 5b-2 of the front 
rotary shaft 5 is shown to be square in cross section, while the engaging 
portion 40c-2 of the box member 40a is in a shape capable of receiving and 
fitting around the shape of the connecting portion 5b-2. This connection 
system allows the connecting portion of the rotary shaft 5 to be connected 
to or disconnected from the engaging portion 40c-2 only when the engaging 
portion 40c-2 thereof is directed in an upward direction. 
In instances where the connection systems as shown in FIGS. 16 to 19 are in 
such a state as capable of connecting the front rotary shaft 5 of the 
front rotation jig 1F to or disconnecting it from the connection shaft 40, 
that is, in such a state that the engaging portions 40c-1 and 40c-2 are 
directed in an upward direction, the vehicle body W should be loaded on 
the carriage D so as to allow its roof panel to stand upright. 
Carriage Changing Apparatus 
A carriage changing apparatus is used to change the carriages D in the 
steps P2, P4, P7, P9 and P11 in order to unload the vehicle body W from 
one carriage D and load it on another carriage D. FIGS. 20 to 22 shows one 
example of the carriage changing apparatus. 
As shown in FIG. 25, the carriage changing apparatus is disposed in a 
loading/unloading station S1 where the locus R1 of conveyance of carriages 
in the previous step is approaching to the locus R2 of conveyance of 
carriages in the subsequent step. The carriage changing apparatus is shown 
to comprise basically a lifter 51 which includes a pair of guide posts 52, 
52 with supporting bases 53 mounted on the guide posts 52 in such a manner 
as operatively moving upwardly or downwardly. The supporting base 53 is 
provided with a supporting arm 54 that is driven so as to extend or 
contract in a horizontal direction. The supporting arm 54 is provided with 
a pair of supporting portions 54a in a spaced relationship along the line 
of the conveyance of the carriage D. 
When the carriage D with the vehicle body W loaded thereon is conveyed from 
the previous step to the loading/unloading station S1 and the carriage D 
is suspended. As the carriage D stopped, the supporting arms 54 are 
extended from the supporting bases 53 located at the lower end positions 
underneath the vehicle body W. The supporting bases 53 are then raised so 
as to allow supporting members 54a of the supporting arms 54 to support 
the floor frame or side sill portions of the vehicle body W and further 
moved upwardly to raise the vehicle body W from the carriage D. The 
vehicle body W is further raised to positions sufficient high for the 
carriage D to be evacuated from the lifter 51 in the loading/unloading 
station S1, as shown by the solid lines in FIGS. 20 and 21. Thereafter 
another carriage D is allowed to enter into a predetermined position in 
the loading/unloading station S1 for loading the vehicle body W currently 
held by the supporting arms 54. As the new carriage D stopped, the 
supporting base 53 is then lowered to reload the vehicle body W thereon by 
transferring the vehicle body W from the lifter 51. The supporting arms 54 
is then lowered to a lower position and then contracted to positions 
closer to the supporting bases 53, as shown by the broken lines in FIG. 
21, in order not to interfere with the movement of the carriage D and with 
entry of another carriage D that carries another vehicle body W for 
unloading. 
It is preferred that the carriage D is fixed unmovably at the predetermined 
position by clamping it from every direction by means of a position 
apparatus or the like while the vehicle body W is being loaded or 
unloaded. 
The carriage changing apparatus may have hangers disposed at its upper 
position so as to be movable intermittently. In this case, the vehicle 
body W may be shifted from the lifter 51 to the hanger, and the hanger 
then convey the vehicle body W to a new lifter 51. The vehicle body W is 
then transferred from the hanger to the new lifter and loaded on a new 
carriage D. 
Spraying of Paints 
The paints are sprayed on the vehicle body W in the intercoating step P3 
and in the overcoating step P10. The spraying procedures to be used in 
these steps are substantially identical to each other so that a 
description will be made on the step P10. 
As shown in FIG. 29, a coating line where the vehicle body is overcoated 
may be divided into eight stations I to VIII, inclusive, in this order in 
the direction of conveyance of the vehicle body W. 
Station I 
The station I is disposed to transfer vehicle bodies W from the continuous 
conveyance system to the tact conveyance system. In this station, the 
vehicle body W is maintained in such a state that a bonnet 95 and a boot 
lid 96 are kept open for coating an engine room 90 and a trunk room 94, as 
shown in FIG. 30. 
As it is not necessary to rotate the vehicle body W in this station, a 
carriage D is shown in FIG. 30 to be of conventional type having no 
rotating mechanism. 
Station II 
The station II is a first-stage coating station for interior coating. As 
shown in FIGS. 29 and 30, a first center coating robot 81 is disposed at 
an intermediate position of the station II along the conveyance line of 
the carriage D extending longitudinally in the middle of the station II. A 
second center coating robot 82 is likewise disposed at the opposite corner 
position of the station II and diagonally across the conveyance line from 
the first center coating robot 81 in such a manner as juxtaposing the 
vehicle body W carried over by the carriage D being conveyed on the 
conveyance line. The first and second center coating robots 81 and 82, 
respectively, are arranged each so as to spray the paint on one quarter 
area or quadrant of an intermediate portion of the vehicle body W, said 
intermediate portion being separated into four areas as divided into 
quadrants by the intersection of the X-axis extending perpendicular to the 
longitudinal direction of the vehicle body W and the Y-axis extending 
parallel to the longitudinal direction thereof. A subsequent description 
on the same and related expressions should be read with reference to this 
definition. 
As shown specifically in FIG. 29 as an example, the first center coating 
robot 81 is arranged so as to spray the paint on a first quarter area or 
quadrant a1, as hatched in the figure, at the forward right-hand section 
of the intermediate portion of the vehicle body W, and the second center 
coating robot 82 is arranged so as to spray it on a second quarter area or 
quadrant a2, as hatched in the figure, at the rearward left-hand section 
thereof diagonal of the first quarter area a1. 
A first corner coating robot 83 is disposed at the other corner position of 
the station II in a line with and forward of the second center coating 
robot 82 and facing the first center coating robot 81 across the 
conveyance line. A second corner coating robot 84 is disposed at the 
remaining corner position diagonal across the conveyance line to the 
opposite corner position thereof and in a line with the first center 
coating robot 81 and facing the second center coating robot 82. The first 
and second corner coating robots 83 and 84 are arranged so as to coat one 
half area of forward and rearward portions of the vehicle body W, 
respectively. The first corner coating robot 83 is to spray the paint on 
one half area c1, as hatched in the figure, at the left-hand position of a 
forward portion of the vehicle body W forward of the intermediate portion 
thereof, and the second corner coating robot 84 is to spray the paint on 
one half area c2, as hatched therein, at the right-hand position of a 
rearward position of the vehicle body W rearward of the intermediate 
portion thereof. 
Station III 
The station III is a second-stage coating station for interior coating and 
is to coat the remainder of the vehicle body W conveyed from the station 
II. As shown specifically in FIG. 29, the third center coating robot 85 is 
arranged at an intermediate position of the station III in a line with the 
second center coating robot 82 disposed in the station II so as to spray 
the paint on a forward left-hand quarter area a3, as hatched in the 
figure, of the intermediate portion of the vehicle body W, on the one 
hand. The fourth center coating robot 86 is arranged at the rearward 
right-hand corner position in a line with the first center coating robot 
81 disposed in the station II such that it sprays the paint on a rearward 
right-hand quarter a4, as hatched therein, of the intermediate portion 
thereof, on the other hand. 
As shown in FIG. 29, third and fourth corner coating robots 87 and 88, 
respectively, are likewise disposed at the opposite corner positions of 
the station III across the conveyance line of the carriage D. The third 
corner coating robot 87 is arranged at the forward right-hand corner 
position thereof in a line with the fourth center coating robot 86 so as 
to spray the paint on a forward right-hand half area c3 of the forward 
portion forward of the intermediate portion of the vehicle body W. The 
fourth corner coating robot 88 is arranged at the rearward left-hand 
corner position thereof diagonal from the third corner coating robot 87 
across the conveyance line thereof such that it sprays the paint on a 
rearward left-hand half area c4 of the rearward portion rearward of the 
intermediate portion thereof. 
The stations II and III are disposed to subject doors 91, 92, the engine 
room 93, and the trunk room 94 to interior coating. The robots 81, 82, 85 
and 86 are provided with door opening or closing means (not shown). 
The arrangement for the center and corner coating robots in the stations II 
and III permits coating by efficiently spraying the paints on the inside 
of the vehicle body W without interfering in movement with each other. 
Station IV 
In this station IV, the vehicle body W conveyed from the station III is 
transferred to a continuous conveyance system from the tact conveyance 
system. In this station, the vehicle body W is subjected to a so-called 
correction coating by manual operation. This correction coating is 
effected mainly on boundary areas between inner and outer portions of the 
body. The touch-up jigs 80 are withdrawn in this station and lock jigs 
(not shown) are mounted to lock and fix doors 91 and 92, the bonnet 95 and 
the boot lid 96 in order to cause them not to open as the vehicle body W 
is turned about the horizontal axis in the subsequent step. 
Station V 
This station is provided to effect exterior coating of the vehicle body W. 
In this station, a top surface and side surfaces are sprayed with the 
paint. The paint is sprayed in two installments using an automatic coater 
of the fixed type or of the reciprocating type. This station is to share 
the spraying of the paint in the first installment and the paint is 
sprayed in an amount less than its sagging threshold value, viz., in such 
an amount as forming a sufficiently thin film without causing any sags. 
Station VI 
This station is to make an expected correction by manual coating. The 
expected correction is made to manually spray the paint in advance on 
expected portions where a correction would be required after exterior 
coating in the subsequent stations VII and VIII. This expected correction 
helps convey the vehicle body W as quick as possible to the following 
setting step after spraying the paint to a film thickness thicker than its 
sagging threshold value in the station VII and VIII. It may be possible to 
carry out this correction coating prior to the station V. 
Station VII 
This station is to effect exterior coating on the outer surfaces other than 
the top and side surfaces of the vehicle body W. The coating in this 
station is made using coating robots 100 and 101 which are disposed at the 
opposite sides of the coating line so as to juxtapose the vehicle body W 
to be coated. The coating on these portions is made once so that the paint 
is sprayed in an amount larger than its sagging threshold value. 
Station VIII 
The station VIII is to effect the second coating on the top and side 
surfaces of the vehicle body W. Spraying the paint in the second 
installment is effected in substantially the same manner as in the station 
V using automatic coating machines. The paint is sprayed here to become 
larger than its sagging threshold value. 
The automatic coating in the stations V and VIII may be preferably carried 
out while turning the vehicle body W in order to minimize the number of 
coating guns. By rotating the vehicle body about its horizontal axis, the 
coating can be effected by spraying the paint thereon from the coating 
guns in one direction because the rotation permits automatical changes of 
the surfaces of the vehicle body to be coated. 
Referring now to FIGS. 23 to 25, there is shown an example of spraying the 
paint from the upper position only. In this case, a mounting bar 57 is 
fixed on the ceiling of a coating booth and plural coating guns 58 are 
mounted on the mounting bar 57. The coating guns 58 are disposed by their 
nozzles or openings facing downwardly the vehicle body W. This arrangement 
permits coating the top and side surfaces of the vehicle body W without 
changing directions of the nozzles of the coating guns 58. For instance, 
the vehicle body W held so as to be rotatable horizontally with its top 
surface directed upward by the carriage D conveyed into the coating booth 
is first sprayed on the top surface thereof with the paint by the coating 
guns 58 as shown in FIG. 23. The vehicle body W is then turned at 
90.degree. about the rotation axis l so as to allow its right side surface 
to face the coating guns 58 whereby the right side surface is sprayed as 
shown in FIG. 24. The vehicle body W is further turned at 180.degree. 
about the rotation axis l and the left side surface of the body is sprayed 
as shown in FIG. 25. 
The coating guns 58 may be disposed so as to spray the paint from a 
transverse direction only. As shown in FIGS. 26 to 28, the coating guns 58 
are mounted on the mounting bar 57 disposed on the right-hand side of the 
coating booth. The vehicle body W is sprayed at its left side surface with 
the paint as shown in FIG. 26. The vehicle body W is then turned at 
90.degree. about the rotation axis l so as to allow its top surface to be 
directed toward the coating guns 58 mounted at the left-hand side portion 
of the booth as shown in FIG. 27. After completion of the coating on the 
top surface, the vehicle body W is further turned at 90.degree. and the 
right side surface of the vehicle body is coated as shown in FIG. 28. 
It is to be understood that the present invention should be construed as 
being not limited to the embodiments described hereinabove and including 
variaties or modifications derived therefrom. The variations or 
modifications may include the following procedural manners as 
illustrative. 
In removing dust from the vehicle body W in the step P1, the vehicle body W 
may be turned about its rotational axis l in such a series as have been 
shown in FIGS. 2(a) to 2(i). Dust may be removed by causing it to fall 
down from the vehicle body W by means of the force of gravity as the 
vehicle body W is rotated so as to allow its inner surfaces where dust 
adheres or scatters to be turned inside down. This treatment prevents the 
falling of dust in the setting and baking steps that follows in which the 
vehicle body W is caused to rotate, thereby ensuring formation of a coated 
surface to which no dust adheres. 
Switching from the rotation of the carriage D to the suspension thereof or 
vice versa or changing of the rotational directions of the vehicle body W 
may be effected by the following procedures regardless of the running or 
suspending of the carriage D. As shown in FIG. 7, there may be disposed 
first and second pairs of chains, which correspond to the chain 34 in the 
figure, so as to allow them to engage each with a sprocket 33 from the 
opposite side in the longitudinal direction. The pairs of the chains are 
to be operatively driven in convenient manner. With this arrangement, the 
rotation of the vehicle body W may be controlled according to the 
following operation modes: 
In the mode in which both the first chains are suspended while the second 
chains are released free, the vehicle body W is allowed to rotate in one 
direction as the carriage D runs. 
In the mode in which both the first and second chains are suspended, the 
vehicle body W is allowed to rotate in the direction opposite to that in 
the above mode as the carriage D runs. 
In the mode where both the first and second chains are released free, the 
vehicle body W is not allowed to rotate in either direction as the 
carriage D runs. 
In the mode in which the first chains are driven in one direction and the 
second chains are released free, the vehicle body W is allowed to rotate 
in one direction as the carriage D caused to stop. 
In the mode in which the first chains are driven in the opposite direction 
and the second ones are released free or where the first chains are 
released free and the second ones are driven in one direction, the vehicle 
body W is allowed to rotate in the direction opposite to that in the mode 
as described immediately hereinabove, as the carriage D is suspended. 
In the above case, a rack bar may be used in place of the chains. In 
instances where the rack bars are disposed always in a fixed state, they 
may be arranged at a constantly or arbitrarily spaced relationship. With 
this arrangement for the rack bars, the vehicle body W may be allowed to 
rotate in arbitrary directions according to the position at which the 
carriage D runs or the rotation of the vehicle body W may be suspended at 
arbitrary positions. 
The rotation of the vehicle body W may be carried out only in the baking 
step of the drying procedures. 
In the case that a two-liquid setting paint is employed in the steps P3 and 
P10, sagging is caused in the setting steps P5 and P12 in the drying 
procedures so that, in this case, the rotation of the vehicle body W may 
be appropriately carried out in the setting steps only. In the case that a 
powder paint is used therein, no setting steps are required so that the 
vehicle body W may be conveniently rotated in the baking steps alone. 
As the substrate, there may be used any material other than the vehicle 
bodies. 
It is to be understood that the foregoing text and drawings relate to 
embodiments of the invention given by way of example but not limitation. 
Various other embodiments and variants are possible within the spirit and 
scope of the invention.