REPAIR OF AIRCRAFT SANDWICH 

CONSTRUCTIONS 

May 1948 

This Report is One of a Series 

Issued in Cooperation with the 

ARMY-NAVY-CIVILCOMMITTEE 

on 

AIRCRAFT DESIGN CRlTERlA 

Under the Supervision of the 

AERQNAUTICAL BOARD 

No. 1584 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5, Wisconsin 

In Cooperation with the University of Wisconsin 
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REPAIR OF AIRCRAFT SANDWICH CONSTRUCTIONS1 

By ED. PANEK, Technologist 

and B. G. HEEBINK, Engineer 

Introduction 

The purpose of this study was to attempt to devise repair techniques 

for various sandwich type constructions. The work was done at the Forest 

Products Laboratory in cooperation with the Army-Navy-Civil Technical Subcommittee on Sandwich Construction. 

Three facing materials aluminum, glass-cloth fabric, and plywoodand five core materials cellular cellulose acetate, cellular hard rubber, 

end-grain balsa, glass-cloth honeycomb, and paper honeycomb were used in 

the study. 

Repair work was divided into three parts: A, damage to facings only; 

B, damage to one facing and core; and C, fracture of both facings and core, 

Parts B and C were further subdivided as to accessibility from one or both 

sides. 

The work was restricted to flat panels with the exception of the glasscloth - glass-cloth honeycomb combination on which some work was done on 

single-curvature panels having a radius of 30 inches. 

Repair Techniques 

General 

The repairs were developed with the objective of equaling, as nearly 

as possible, the strength of the original panel. In order to eliminate 

dangerous stress concentrations, abrupt changes in cross-sectional area were 

avoided wherever practicable by tapering joints, making small patches round 

or oval-shaped instead of rectangular, and rounding corners on all large 

patches. Because smoothness of outside surfaces of high-speed aircraft is a 

necessity for proper performance, patches that projected above the original 

surfaces were avoided whenever practicable. When this was impossible the 

1This progress report is one of a series prepared and distributed by the 

Forest Products Laboratory under U. S. Navy, Bureau of Aeronautics No. 

NBA-PO-NAer 00619 Amend. No. 1, and U. S. Air Force No. USAF-PO­

(33-038)48-41E, Results here reported are preliminary and may be revised 

as additional data become available, 

Report No. 1584 -1-
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edges were tapered. Uniformity of thickness and the continuity of the core 

are also necessary where radar equipment is involved, Repairs involving 

punctured facings and fractured cores therefore necessitated removal of all 

of the damaged material and its replacement with the same type of materials 

and in the same thicknesses as the original. 

Damages were assumed to vary in size from small holes the size of a 

pencil to large holes extending over several square feet. Three types of 

patches round, oval, and square or rectangular were devised to repair 

holes of various sizes. 

Cutting and treatment of the core were done according to methods 

described in Forest Products Laboratory Report No. 1574.2 

Plywood-faced Panels 

Repair tools and equipment, discussed in Sections IV and V of 

AN-01-1A-7, "Repair of Wood Aircraft Structures," November 1944, were found 

to be applicable. 

Scarf joints having a slope of 1 in 12 were used for repairs to the 

facings. Pressure was applied to the joint by No. 5 roundhead wood screws 

driven through 1/4-inch plywood backing strips or pressure plates. Grain 

direction of the patch matched that of the original. A room-temperaturesetting resorcinol glue, adhesive D,3 was used for all patches. Two rows Of 

screws were used on the round and oval patches. The first row followed a 

line drawn around the hole in the center of the scarf. The second row was 

3/8 inch in from the edge of the hole. Screws in the rows were spaced 1 inch 

apart. For the square patch, four rows of screws were used. Suggested 

details are shown in figures 1, 2, and 3. The steps in making an oval repair 

when one side only is accessible are shown in figure 4. 

No satisfactory method was found to repair damages to one facing and 

core when only one facing of the panel was accessible. Damage of this nature 

was treated the same as damage where both facings and core were damaged. 

Aluminum-faced Panels 

The technique for the oval patch when the core and both facings were 

damaged was developed (figs. 5 and 6), but the bonds made with the glues 

available were erratic in quality. Further work, therefore, was stopped pending development of a better glue for bonding metal to metal at low pressures 

(vacuum pressure) and at temperatures of 250° F. or less. To date, however, 

no such glue has been made available for test. 

2"Fabrication of Lightweight Sandwich Panels of the Aircraft Type," Forest 

Products Laboratory Report No. 1574, June 1947. 

3Appendix I, Note 4. 

Report No. 1584 -2-
The details of making an oval-shaped repair when only one side of the 

panel is accessible are shown in figure 7. 

Cutting of the panels was accomplished with a plug cutter on round 

Cuts, using a template whenever possible, and with a reciprocating-saw attachment to a 1/4-inch electric drill, using a 24-tooth hacksaw blade, on 

straight cuts. The hole was filled with a plug made of the same material and 

having the same thickness of core and facings as the original. Plates were 

glued to both faces. A hot-press thermosetting glue, adhesive E,-4 was used 

for the patches. 

All surfaces to which glue was applied were etched. They were first 

sanded lightly with 320A paper, then cleaned with acetone. Etching of the 

plates was done by submerging in a hot sulfuric acid-sodium dichromate bath 

(10 parts by weight of concentrated sulfuric acid, 1 part of sodium dichromate, 

and 30 parts of water) for 20 minutes at 145° F. The plates were rinsed and, 

if necessary, rerinsed until a smooth unbroken water film was obtained. The

panel and plug covers were etched by swabbing cold (75° F.) sulfuric acid -

sodium dichromate solution on the faces, taking precautions to prevent any of 

it from getting on the core. Fresh solution was added several times during 

the treatment, and after 30 minutes it was removed by wiping with a clean 

cloth saturated with water and dried with a clean dry cloth. 

After etching, two coats of adhesive E4 were applied to all surfaces. 

One hour was allowed between coats at 75° F., and 16 to 24 hours were allowed 

between the last coat and curing. When both faces were accessible, 0.012-inch 

24ST aluminum plates 1-1/2 inches wider on all edges than the hole were glued 

to both faces. The patch was applied in one operation, using a heated vacuum 

blanket on each side to supply pressure and heat. 

When only one side was accessible, a backing plate of 0,064-inch 24ST 

aluminum was used on the back and a 0.012-inch 24ST aluminum plate was used 

on the front face. The edges of the 0.012-inch plates were tapered with a 

file. Three major steps were required to complete the patch. First, the 

backing plate was glued to the back face, No. 8 self-tapping screws (sheetmetal screws) with lock washers under the heads were used to apply pressure, 

and a rubber electric heating pad supplied the heat. Temperature on the surface of the panel was not allowed to exceed 260° F. At an air temperature Of 

75° F., temperatures of 185° to 190° F. were obtained at the glue line. Heat 

was applied for 3 hours. Second, the sandwich plug vas glued to the backing 

ring. Self-tapping screws were used for pressure as in step one. Heat was 

again applied with an electric heating pad. With the temperature not exceeding 260° F. on the panel surface, temperatures of 175° to 185° F. were obtained 

at the glue line, Heat was again applied for 3 hours. Third, the front cover 

was glued to the plug and original facing, A heated vacuum blanket was used 

for pressure and heat for a period of 3 hours, again with the temperature on 

the face of the patch limited to 250° to 260° F. 

4

Appendix I, Note 5. 

Report No, 1584 -3-
Screw holes drilled through the facings and plug were equal in size to 

the major diameter of the No. 8 self-tapping screws. The holes through the 

backing plate were equal to the minor diameter of the screws. Three rows Of 

screws were used. The first row was located 3/8 inch and the second row 1-1/8 

inches in from the edge of the backing plate. The holes were spaced 1-1/2 

inches apart in the rows. The first two rows of screws were used when the 

backing ring was glued to the back cover. The third row was used when the 

plug was glued to the backing plate. The screws were long enough to pass 

through the backing plate and project 1/4 inch beyond. Screw-hole location 

and drilling were done on the backing ring first. The backing ring was used 

as a template to locate the screws on the plug, panel, and pressure ring. 

Before the front face plate was applied, all screw holes were filled with 

patching putty.5 A 4-hour drying period was allowed before the glue was 

spread on the top facing. 

Glass-cloth-faced Panels 

Several attempts were made to make repairs to glass-cloth-faced panels 

when only one side was accessible, but no satisfactory technique has been 

devised to date. A repair technique for the oval patch when both sides were 

accessible was developed. The results were somewhat erratic, however, due 

primarily to the quality of the scarfing, The method used for scarfing the 

faces was not too successful. No better method has been tried to date. 

Suggested details for the preparation of an oval repair when both 

sides are accessible are shown in figure 8, and the steps in making this 

repair in figure 9. 

When round cuts were necessary in removing the damaged portion of a 

panel, a plug cutter in a hand brace was employed. A template was used 

whenever possible. Straight cuts were made with a 24-tooth hacksaw blade in 

a reciprocating-saw attachment on a 1/4-inch portable electric drill. The 

facings were scarfed for a distance of 2 inches around the hole with a 

flexible disk sander using medium-grit cloth. Extreme care was taken to get 

the scarf uniform and not to gouge out pieces of the core. The core plug was 

cut from material having the same thickness as the original core. Face 

patches were made using the same number of plies as were used in the original 

face. Starting with a ply the size of the opening, each successive ply was 

slightly larger with the final ply 2 inches larger on all sides. Before 

cutting the patches, the glass cloth was impregnated with a long-assembly, 

high-viscosity resin, such as resin A,6 which was used for this study. 

Methods for impregnating the cloth are discussed in Forest Products Laboratory Report No. 1574.2 The impregnated cloth was stretched out and laid on a 

sheet of cellophane with a second sheet of cellophane laid on top of the 

cloth to facilitate handling and storing. The outlines of the patches (two 

of each size) were then drawn on the cellophane, and the patches cut with a 

pair of 9-inch office shears. 

5 Appendix I, Note 11. 

6Appendix I, Note 1. 

Report No. 1584 -4-
Cellophane was placed on a sheet of smooth aluminum, both slightly 

larger in dimensions than the largest patch, The cellophane was removed from 

one side of the largest patch, which was then laid (exposed glass-cloth face 

down) on the cellophane-covered sheet of aluminum. The cellophane was 

removed from the top side of the patch and all. wrinkles and air bubbles were 

worked out. This procedure was repeated until the entire patch was laid up. 

Care was taken to center each ply. The whole assembly was then removed from 

the sheet of aluminum by cutting the cellophane with a knife leaving a 1-inch 

border around the patch, Before positioning the patch on the panel, the 

scarfed portion of the panel around the hole was coated by brush with the 

same resin as that used for Impregnating the cloth to a spread of 10 grams 

per square foot. When necessary, as in the case of glass-cloth honeycomb. 

the resin was also applied to the core plug. The patch was then positioned 

over the core plug with the glass-cloth face down. The air bubbles were 

worked out of the cover with the hands and the cellophane was punctured at 

about 1-inch spacing with a needle, The procedure was repeated for the patch 

on the opposite side. Electrically heated vacuum blankets were taped to both 

sides over the patches. Vacuum was applied and the temperature was raised to 

125° F. for 10 minutes, then increased to 230° F. and held there for 1 hour. 

An additional step, found applicable only when the sandwich had a 

honeycomb core, was to use a thin (0.005- to 0.012-inch) aluminum caul over 

the cellophane. This caul gave a smoother outer surface. 

Attempts were made to repair a curved panel having a 30-inch radius 

but the results were not satisfactory, Blisters developed in the patches, 

especially on the concave side, due probably to the thermal distortion of the 

0.005-inch aluminum cauls used. In later trials the aluminum caul was 

eliminated from the concave side; but, although results were somewhat more 

satisfactory, blisters were not entirely eliminated. 

Test Procedures and Test Results 

Glue and Resin Selection 

Plywood facings.--A room-temperature-setting resorcinol glue, 

adhesive D,3 was selected and used for all patches. A spread of 35 grams 

per square foot of glue line was used, one-half applied to each contacting 

surface. 

Aluminum facings.--Tension tests were made on 0.012-inch 24ST Alclad 

aluminum. Two pieces 6-3/4 inches by 12 inches were joined together using a 

1-1/2-inch lap joint. Both pieces were first sanded lightly with 320A paper 

and cleaned with acetone, then etched. Piece No. 1, representing the standing 

part of the plane, was etched by the cold method. Piece No. 2. representing 

the patch, was etched in the sulfuric acid-dichromate hot bath. Glue was 

spread on both surfaces to be joined, and the two pieces were assembled on a 

smooth sheet of aluminum with cellophane placed on top and beneath the joint. 

A heated vacuum blanket was taped down over the assembly, and heat and 

pressure were applied for a specified period, depending on the glue used. 

Report No. 1584 -5-
The glues or combinations of glues tested follow: 

(1) Adhesive F7 + adhesive G8 with 6 percent accelerator9.--Adhesive F 

vas spread as a primary costing on both surfaces with a brush. Adhesive F on 

piece No. 1 was cured under a heating pad at about 275° F. for 1 hour. Adhesive F on piece No. 2 was cured in an oven at 325° F, for 15 minutes. Both 

surfaces were sanded lightly with 320A paper to remove irregularities. Then 

adhesive G was spread as a secondary coating with a brush at a rate of 10 

grams per square foot on each surface. 

room temperature for 1 hour. The pieces were assembled and cured under a 

Adhesive G was allowed to air dry at 

heated vacuum blanket at about 230° F. for 1 hour. 

(2) Adhesive F7 + adhesive G8 with 20 percent accelerator9.--Same 

procedure used as in (1), except cure was accomplished at room temperature 

(75° F.) in 8 hours. 

(3) Adhesive H10.--The adhesive H liquid was spread with a brush at a 

rate of 20 grams per square foot on each surface. While the liquid was still 

wet, the thermoplastic powder was sifted on and, after drying, the excess was 

removed. The joints were cured under a heated vacuum blanket for 1 hour at 

275° F. 

(4) Adhesive E4.--Adhesive E was thinned 50 percent with adhesive E 

thinner11 and brushed on both surfaces. Two coats were applied, allowing 1 

hour between coats and 16 to 24 hours between the second coat and curing. 

Dry-film thickness varied between 0.004 to 0.006 inch. The joints were cured 

under a heated vacuum blanket for 1-1/2 hours at 275° F. 

(5) Forest Products Laboratory Glue No. M-249 (Adhesive I12) .--Two 

coats of glue M-249 were applied to each surface with a brush, allowing 1 hour 

air drying at room temperature, open assembly between coats and 1 hour between 

second cost and curing. The joints were cured under a heated vacuum blanket at 

250° to 260° F. for 2 hours. The dry-film thickness was 0.004 to 0.006 inch­

(6) Forest Products Laboratory Glue M-432 (Adhesive J13) .--Two coats 

of glue M-432 were applied with a brush allowing 2 hours of air drying at room 

temperature between coats and 30 minutes between second coat and curing. The 

joints were cured under a heated vacuum blanket at 250° to 260° F. for 15 

minutes. The dry film thickness was approximately 0.005 inch. 

After curing, five 1- by 12-inch specimens were cut from each panel 

and tested in a universal testing machine. Results are shown in table 1. 

7Appendix I, Note 6. 

8Appendix I, Note 7. 

9Appendix I, Note 13. 

10Appendix I, Note 8. 

11Appendix I, Note 14. 

12Appendix I, Note 9. 

13Appendix I, Note 10. 

Report No. 1584 -6-
All bonds were somewhat spotty, indicating a lack of adequate pressure. 

The adhesive H10 bond was the poorest, as indicated by the joint efficiency 

of 58.5 percent. The adhesive E4 bond, although spotty, was the best of the 

six glues tested, as indicated by the joint efficiency of 90.7 percent. 

Although the adhesive E bond was not considered entirely satisfactory, 

it was used for the repair of the metal-faced panels, 

Glass-cloth faces. --A high-viscosity resin having a long assembly time 

was found to be most suitable for repair work. Glass cloth impregnated with 

a low-viscosity resin having a short assembly time usually was fairly dry and 

stiff before the lay-up of the patches was completed and usually resulted in 

blistered patches. 

Since it is often impossible to determine what resin was originally 

used in fabricating the panel, a compatability test was conducted to check 

whether or not a resin would cure in the presence of the same or a different 

cured resin. 

Panels (12- by 12-inch) were bag molded of 10 plies of glass cloth14 

impregnated with one of the following resins: resin A,6 resin B,15 or 

resin C.16 Resin A and resin B are high-viscosity, long-assembly resins, 

while resin C is a low-viscosity, short-assembly resin. These panels were 

cut in half and three 1- by 12-inch specimens were cut from one of the pieces 

for controls. The second half was again divided into two equal parts and one 

edge of each of the 6- by 6-inch pieces was scarfed with an accurate machinecut scarf for a distance of 2 inches. 

Six- by 8-inch patches were wet-laminated to the scarfed edge using 

cloth impregnated with a high-viscosity, long-assembly-time resin. The 

scarfed edge was first coated with the same resin used for impregnating the 

cloth applied at a rate of 10 grams per square foot. Then the first ply of 

the patch was butted against the lower edge of the scarf. This was found to 

be necessary in actual repair to produce uniform thickness across the patch. 

The other nine plies of glass cloth were spaced equidistant along the scarf, 

each being placed farther up with the last ply approximately 2 inches from 

the edge. The lay-up was made on a sheet of aluminum covered with cellophane. 

A second sheet of cellophane was placed over the assembly and perforated with 

pin holes. A heated vacuum blanket was then taped down over the assembly. 

Vacuum pressure was applied and the temperature raised to 125° F. After 10 

minutes it was raised to the curing temperature and held for 1 hour. The 

curing temperature for resin A6 was 230° F., and for resin B15 ft was 240° F. 

After curing, the panel was trimmed to 6 by 12 inches and three 1- by 12-inch 

specimens were cut. These specimens along with the three control specimens 

were tested in tension on a universal testing machine. The results are given 

in table 2. 

14Appendix I, Note 12. 

15Appendix I, Note 2. 

16Appendix I, Note 3. 

Report No. 1584 -7-
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An examination of the broken specimens revealed no indication of noncompatibility, as all of the failures were tension breaks at the end of the 

scarfed piece. A few additional tests on specimens made by overlapping all 

the plies (instead of the butt joint between the edge of the first ply) 

revealed an increase of about 10 percent in joint efficiency. This increase 

in strength could be realized if thickness of facing was not the primary 

consideration. 

Repair Strength Tests 

Oval patches 2-1/2 by 8 inches in size were cut in panels 24 by 24 

inches in size and repaired by the recommended procedures. All cores were 

1/4 inch thick. Facing thickness varied with the material: A, plywood 

0.070-inch three-ply birch manufactured in accordance with Specification 

AN-P-69a; B, aluminum 0,012-inch 24ST Alclad; C, glass cloth14 six 

plies, cross laminated. Preparation of the core, face material, and fabrication of the panels was in accordance with methods described in Forest Products 

Laboratory Report No. 1574.2 

Sandwich combinations used in the study were as follows. 

Patches made when both sides were accessible: 

Plywood - End-grain balsa 

Aluminum - End-grain balsa 

Aluminum - Paper honeycomb17 

Glass cloth - Cellular hard rubber18 

Glass cloth - Cellular cellulose acetate19 

Glass cloth - Glass-cloth honeycomb20 

Glass cloth - End-grain balsa 

Patches made when only one size was accessible: 

Plywood - End-grain balsa 

Aluminum - End-grain balsa 

Aluminum - Paper honeycomb17 

Three panels were used for each combination for each type of repair. Tension 

and compression specimens were cut (fig. 10) and tested in a universal testing machine. Compression specimens, 2 by 3 inches, were substituted for the 

1- by 4-inch specimens shown in figure 10 in all of the aluminum-faced panels 

and the plywood-faced panels where only one side was accessible, The results 

are recorded in table 3. 

17Appendix I, Note 15. 

18Appendix I, Note 16. 19Appendix I, Note 17. 

20Appendix I, Note 18. 

Report No. 1584 -8-
Plywood-faced specimens usually failed in the patch either over the 

plug or in the scarf joint. Aluminum-faced specimens usually failed in the 

glue joint. The glass-cloth-faced specimens failed at or near the outer 

edge of the scarfed joint. The joint strength values for the glass-clothfaced panels, as shown in table 3, were somewhat erratic due mostly to the 

quality of the scarfing operation. Where the scarfing exceeded the 2-inch 

limit a decrease in strength proportional to the decrease in thickness of 

the faces resulted. 

Uniformity in thickness is essential to the satisfactory performance 

of a sandwich radar housing; therefore, repairs to these housings are subject to the same close thickness tolerances. Thicknesses of typical repairs 

and panels were measured at the stations shown in figure 11. These measurements, presented in table 4. show a greater variation in thickness across 

the repaired section, in most cases, than across the panel. The maximum 

variation in the case of a repair to the glass-cloth - glass-cloth-honeycomb 

combination was 0.038 inch, or about 14 percent, on the patch, and 0.033 

inch, or about 12 percent, on the panel itself. On the glass-clothcellular-hard-rubber combination the maximum variations were about 13 

percent and 7 percent, respectively. In most cases the patch was slightly 

thicker than the panel. 

Patching Putty for Aluminum-faced Panels 

Two unpainted aluminum-faced panels each having four dents filled 

with putty were subjected to a repeating cycle of 8 hours at 200° F. followed 

by 16 hours at -20° F. This cycle was repeated 20 times. No deterioration 

of the putty was observed at the end of the period, The panels were then 

exposed to 80° F. and 97 percent relative humidity for 3 months. By the end 

of this exposure scaling of the putty occurred where it was less than 1/32 

inch thick, apparently due to tho breakdown of the putty and to the 

corrosion of the metal. 

Report No. 1584 -9-
APPENDIX I 

Description of Resins and Adhesives. 

Note 1. Resin A. A high-temperature-setting, high viscosity, contactpressure, laminating resin of the polyester type. 

Note 2. Resin B. A high-temperature-setting, high-viscosity, contactpressure, laminating resin of the polyester type. 

Note 3. Resin C. A high-temperature-setting, low-viscosity, laminating 

resin of the styrene monomer, polyester type. 

Note 4. Adhesive D. A room-temperature-setting resorcinol resin. 

Note 5. Adhesive E. A high-temperature-setting mixture of thermoplastic 

and thermosetting resins. 

Note 6. Adhesive F. A high-temperature-setting mixture of thermosetting 

resin and synthetic rubber. 

Note 7. Adhesive G. A high-temperature-setting, acid-catalyzed, phenolic 

resin. 

Note 8. Adhesive H. A high-temperature-setting, two-component resin with 

a thermosetting liquid and thermoplastic powder. 

Note 9. Adhesive I. A high-temperature-setting mixture of thermoplastic 

and thermosetting resin. Developed at the U. S. Forest Products 

Laboratory, Madison, Wis. 

Note 10. Adhesive J. A room-temperature-setting mixture of thermoplastic 

and thermosetting resins. Developed at the U. S. Forest 

Products Laboratory, Madison, Wis. 

Note 11. Patching putty. A synthetic putty or crack filler used for filling 

dents in aluminum. 

Note 12. Glass cloth. Heat-cleaned cloth chemically treated with a chrome 

complex bath. 

Note 13. Accelerator. An acid catalyst for use with adhesive G. 

Note 14. Thinner. A specially formulated thinner for use with adhesive E. 

Report No. 1584 -10-
Note 15. Paper honeycomb. Honeycomb core material (approximately 1/8-inch 

cell size) developed at the U. S. Forest Products Laboratory in 

cooperation with the National Advisory Committee for Aeronautics 

Made by impregnating with resin 4.5-mil kraft paper, corrugated 

by passing between B-flute corrugating rolls, as described in 

Forest Products Laboratory Report No. 1574.2 

Note 16. Cellular hard rubber. A core meterial, black in color, 8 pounds 

per cubic foot density (including skin). 

Note 17. Cellular cellulose acetate. An extruded, unoriented, multicellular form of cellulose acetate containing a small percentage 

of glass fiber as a filler, 7-8 pounds per cubic foot density 

(including skin). 

Note 18. Glass-cloth honeycomb, Honeycomb core material with 3/16-inch 

hexagonal cells, made by impregnating glass cloth (note 12) with 

resin to give a density of 9 pounds per cubic foot. 

Report No. 1584 -11-
Table 1.--Resultsof tension tests aluminum to aluminum -- 

Table 2.--Results of tension tests on scarfed, wet-laminated 

joints in glass-cloth panels using different resins 

Report No. 1584 



Figure 2.--Oval-shaped patch assembly need in plywood-faced panels 

when one side only is accessible. 

Z M 75914 F 
Figure 3.--Square or rectangular patch assembly used in plywood- faced panels when one side only is accessible. 

Z M 75915 F 
Figure 4.--Steps used in making an oval-shaped scarfed patch in a 

sandwich panel having plywood facings and end-grain balsa core when one side only is accessible. A , cutting holes at the end 

of the oval repair by means of a centerless hole saw in a hand 

brace; B , sawing out the area between the holes; C , spreading

glue on the backing ring to be glued to the back side of the 

panel; D , clamping backing ring to panel; E , scarfing face side of repair. 

Z M 77480 F 
Figure 4 (continued) .--F , alternate method of scarfing plywood,

using an electric router in a special adapter; G , underside of 

Z M 77481 F scarfing device showing details of construction. 
Figure 4 (continued) .--H , scarfed face patch prepared (Note back 

face and core filler .) ; I , spreading glue on parts preparatory 

to assembling; J , completing glue-spreading operation on face patch; K , laying cellophane parting film over repair; L , drilling 

screw holes through repair; M , screwing pressure plate in place; and N , scraping glue from finished repair.

Z M 77482 F 

Figure 6.--Oval-shaped patch assembly used in aluminum-faced 

panels when one side only is accessible. 

Z M 75917 F 
Figure 7.--Steps in making an oval-shaped repair in a sandwich 

panel having aluminum facings on end-grain balsa core. A , 

cutting holes at the ends of the oval-shaped hole; B , sawing 

out the damaged area, using a reciprocating-saw attachment 

for an electrical drill; C , removing the burr on the sawn aluminum edges; D, screw holes drilled in area around hole (Note 

aluminum backing plate with finger holes and plywood caul 

ring.) E , backing plate being screwed in place, using selftapping screws through plywood caul ring; F , continuing to screw 

backing plate in place (Protector plate has been dropped in 

place to cover holes in backing plate increasing heat transfer 

to glue line.) 

Z M 77483 F 
Figure 7 (continued) .-- G , electrically heated blanket taped in place 

over patch to provide heat for curing glue; H , plug patch being 

screwed in place, using self-tapping screws through a plywood caul; 

I , heated blanket again taped over patch to cure glue between plug 

and backing plate; J , plugging screw holes with special filler to 

prevent leakage; K , glue spread on patched area and surface plate 

preparatory to gluing surface plate in place; and L , electrically 

heated vacuum blanket in place over patch in final gluing operation 

(Later, glue squeeze-out was cleaned off preparatory to finishing.) 

ZM 77484 F 
Figure 8.--Oval-shaped patch assembly used in glass-cloth-faced 

panels when both sides are accessible. 

Z M 75918 F 
Figure 9.--The steps in making an oval-shaped patch in a sandwich 

panel having glass-cloth facings on glass-cloth honeycomb cores, 

with both sides accessible. A , cutting holes at the end of the 

oval repair by means of a centerless hole saw in a hand brace; 

B, sawing out the damaged area with a reciprocating-saw attach­ ment for an electric drill; C , scarfing the area around the hole, 

using a flexible disk sander; D , core has been laid in place, and 

resin is being applied on scarfed face. 

Z M 77485 F 
Figure 9 (continued).--E, the individual pieces of resinimpregnated, cellophane-covered glass cloth necessary to 

build up the repaired facing. 

M 77486 F 
Figure 9 (continued) .--F , procedure of laying up the patch on a cellophane-covered aluminum caul; G, laying complete face lay-up

in place over area to be repaired; H , pin-pricking the cellophane

covering to aid escape of entrapped air; I , curing both faces of 

repair simultaneously under two electrically heated vacuum 

blankets . 

ZM 77487 F 
Figure 9 (continued) .--J , finished repair. 

Z M 77488 F 

Figure 10.--Cutting diagram for test panels having glass-cloth 

faces. 

Z M 75919 F