Windshield washer fluid(plain blue) works fine, but I have discovered from a Facebook Group a thinner solution that is outstanding for craft paints. It is 3 parts distilled water, 1 part 91% isopropyl alcohol( I'm venturing a guess that the 70% will work fine too, I just haven't tried it yet) and 4-5 drops of glycerin as a flow improver per 8 ounce batch. The glycerin is super slick, so not much is needed. If flaws need to be sanded, it should be done lightly and carefully and will almost certainly need to be re-touched with paint. Adhesion is pretty good, and I do strongly suggest a primer coat first. I have had little to no problems with masking over craft paints. I have sprayed Testors Wet Look Clear, Tamiya TS-13, and Krylon Short Cuts Clear over craft paints with no problems at all. Just start with a light coat or two first before the heavier wetter coats. Practicing/testing any of these questions on a test body and/or parts will give you a really good indication of how craft paints will work/react to your building style. I mainly use craft paints and Createx anymore and love them, there's just a bit of a learning curve. They're cheap, easy to use and clean, have no odor, and are available in about a million colors, lol.