From: c34657b@saha.hut.fi (Tuomas Viljanen)
Subject: Paint drying mechanisms
Organization: The Nibelungenlied Incorporated
Date: Fri, 8 Jan 1993 23:24:20 GMT

These has been discussion whether or not old paints can be used. It depends
much of the paint type and in which conditions it has been stored.

Paints can dry (form a plastic layer over the surface to be painted) in two
ways: either physically or chemically. 

The former means that the binding agent (varnish) of the paint is a thermoplast
and it is as a solution in the paint: when the solvent evaporates, the binding
agent forms a thermoplast film. It can be dissolved again in the solvent, if
desired. an example of this are the acrylics. No chemical reaction occurs.

The latter means that there occurs a chemical reaction as the paint dries.
A thermosetting plastic layer is formed, as the paint is applied. The
film is in the paint in monomer state, either as a solution or as an emulsion
(usually water-based), or it is in two components to be mixed immediately
before applying (like epoxy resin based paints). When the solvent evaporates,
the monomers undergo a reaction between each other and with the oxygen of the
air (the saturation of unsaturated fatty acids in alkyd resins to form cross-
linking between the monomer molecules). This reaction is _irreversible_ and
once the paint has been cured, it cannot be removed by just washing it away
with solvent. The paint has formed a thermosetting plastic, and, in theory,
the whole surface is one huge molecule. Common enamels are an example of this.

So what does this concern on old paint usability ? A great deal. If the paint
is of physically drying kind, the situation is in control, just add solvent and
stir _thoroughly_. The situation is more difficult if the paint is of chem-
ically drying sort: it depends on whether or not the paint in the can has been
exposed on the oxygen of air. If the paint has never been used, the can is
factory sealed and/or nice liquid-like sound is heard when the can is shaken,
there are good chances that the paint is usable. If the lid-can joint has not
been airtight, the paint is likely to be unusable. If, after stirring (perhaps
with a motor tool) the paint can be put in even, running state, it perhaps
can be used, though with caution; if the leakage has been big, it is likely 
that the paint has gone into "irreversible gel" state. In this state the 
varnish has reacted into a net structure, in which the pigment and solvent
is bound in a jelly-like state, it is unusable and to be thrown away. If the
paint, after stirring still resembles an aladobe, throw it away; it has gone
into irreversible gel. Neeedless to sayif the paint has dried thoroughly into
a solid block, it can be used only as nose weight for a trike landing gear
planes, and it is usually too light for even this use.

If your paint is of chemically drying type, the best thing to do with their
storage is to make sure that the cans are airtight. That is, to turn the
threaded caps tightly or to pound the lids of tins in securely... well, you can
seal them  aby casting some molten candle in the concavity between the lid
lip and the tin wall. Another thing you can do is to use plastic photo film
containers as storing vessels; they are airtight and you can easily see the
shade of the paint if they are translucent, like those of Fuji. But always 
remember to clean the edges of the lid and can before closing the lid and seal-
ing it. If the lid level is at higher state than the tin edge level, there are
good chances that the paint will be, or has been, wasted.


--++ Tuomas Viljanen                     ++ For a battle like Crecy, you do ++
  ++ Lahderanta 20 A 19                  ++ not need a military genius like ++
  ++ SF-02720 Espoo FINLAND              ++ Edward III. All you need is an  ++
  ++ 358-0-592175 or c34657b@saha.hut.fi ++ idiot like Duke of Alencon.     ++
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