flat coat

Although Swanny has a wealth of very good and useful information on his site, this is one thing I strongly disagree with. Windex contains ammonia, which plays havoc with acrylic molecules. That's why it works so well to clean them. As I pointed out elsewhere, just because something will dissolve an acrylic paint does not mean that it is a compatible solvent.

You did not mention what brand of paint you are using. It makes a difference to what solvent you should use. When in doubt, use the proprietary thinner.

Here are a few of the substitutes that I know work:

Tamiya, Gunze Sangyo: 90% isopropyl alcohol with acrylic retarder.

Polly Scale: distilled water. Acrylic retarder optional.

MisterKit: distilled water or a 1:1 mix of distilled water and alcohol. Acrylic retarder optional. Note that cured MisterKit is attacked by mineral spirits.

Vallejo (both types): distilled water. Acrylic retarder optional.

The following are from Aaron Skinner's FSM article:

Xtracrylics: distilled water. Acrylic retarder optional.

Model Master Acryl: 90% isopropyl.

Triarius

 As I pointed out elsewhere, just because something will dissolve an acrylic paint does not mean that it is a compatible solvent.

Ross, did you mean to use thinner instead of solvent? I figure if it dissolves the paint and leaves the surface intact, it's a compatible solvent, but it may not mean it's an appropriate thinner.

Just in case people might wonder what the differences are between a solvent and a thinner. A solvent is almost without exception a thinner, but a thinner is not always a true solvent. In the examples above where the paint can be thinned with water, the water is a thinner, it reduces the viscosity by being a diluent. It is not a solvent, as it cannot dissolve the cured paint.

And I agree with you, I use Windex when I want to remove paint, not thin it. You could probably get away with using it in a crunch, but the risk of the ammonia causing adhesion issues is too great for me. Besides, Windex is mostly water, with only about 25% isopropyl alcohol and 3% ammonia. It's cheaper than proprietary thinners because you're buying mostly H2O. That and it's not proprietary!

Now that I'm thinking about it, why can't we use water to thin Tamiya paints? Does the water reduce the viscosity past the point of curing?

No, Bill, I didn't. I was using paint terminology. Technically, a "thinner" is a solvent when used to reduce the viscosity of a paint to a desired number. A "reducer" is a solvent when used to reduce a paint to spraying viscosity. Both are solvents, and to do either task, they must be compatible with the binders in the paint. The term "thinner" is generally misused—I do it simply because lay persons are confused by the proper term, "reducer."

A solvent is not necessarily compatible: isopropyl alcohol is a compatible solvent for Tamiya acrylics. It is a nonpolar solvent. Water is not a compatible solvent for Tamiya, probably because it is a polar solvent. (Actually, it is marginally compatible—doesn't turn the paint to goo or sand, but doesn't do a good job as a diluent or reducer.) Alcohol turns Vallejo acrylics into goo, if it will mix with them at all. It is an incompatible solvent. It is important to remember that solvents fall into one of these three categories. A compatible solvent is one that will disperse the binder and pigment without causing a chemical change in either. A marginally compatible solvent may disperse the binder and pigment poorly, cause minor, reversible changes in the  binder, or may do so if its relative concentration in the paint is too high. An incompatible solvent will either not mix with the binder, or will cause irreversible changes in the binder or pigment, or both. Those irreversible changes to the binder are what Windex or any ammonia cleaner does to acrylic paints.

Viscosity of the paint has no effect on curing, only on drying. Once the solvent evaporates to a critical degree, curing begins, regardless of how much solvent was there in the first place.

Which is why I usually wait until you post and go "Ditto"!

Im sorry the flat coat I have is Model masters I want to use

ronald305

Im sorry the flat coat I have is Model masters I want to use

Then according to Skinner's article, you should try 90% isopropyl alcohol.

thanks