Paint thinners used with enamel paints, to my knowledge, have two purposes: the first is obviously to thin the paint for proper airbrushing or brush painting. The second is as a catalyst to cure the paint once applied to a surface. I recall many years ago reading modeling articles where the author would note that he would add a few drops of thinner to a freshly opened container of paint with the idea that the evaporated thinner must be replaced (!!??). I tried that and found that the unused paint eventually turned to sludge after not being used for several weeks. This would indicate to me that thinner added to a container of enamel paint started the curing process. Other enamel paints "lost" in the paints box, never opened, remained useable.

I use Windsor Newton oil paints for washes in aircraft panel lines. I normally mix the paint at a ratio of 1:10 with odorless thinner, and I have starting to date the container. I have found that after about 3 to 4 weeks the oils turn to tiny granuals in the thinner and no longer can be used as a wash, so I start a new batch.

Over the years I have switched to acrylics, but still refrain from adding thinner to the unused paints, and I generally use the paint before it can go bad, if it would. I still retain quite a stock of enamel paints (ModelMaster) and use them often for hand brushing details and drybrushing. Very seldom do I open one and find a sludge surprize.

I am curious to know if any one else has experienced this with enamels and thinners, and what the recommendations might be. Also, comments on you experience with acrylics would be interesting.

Thanks to all for your help.

Paint thinner's only purpose is to reduce enamels for spraying. Some of the reducer is lost in the atomization process, the rest stays in the paint film long enough to aid flow and then evaporates. The curing mechanism of enamels is oxidation. The resin reacts with oxygen to form a hard film.

The reason paints set up in opened bottles is primarily due to the "head room" between the surface of the paint and the cap. This space holds oxygen and the paint resin reacts with it just as if it was sprayed. The more "head room" the more film forming takes place.

The granules you see in reduced artist oils is "kick out". Paints are primarily made up of pigment, binder and solvent. In the case of artists oils the ratio of binder to pigment is low. When you reduce the oils to make a wash, over time the binder looses the ability to hold the pigment together and then you see separation.