Because Polly Scale, Testors Model Master Acryl and Vallejo paints are clearly water based, I’ve used distilled/deionized water, or tap water, to thin them. As in anything with water-based paints, you start by thoroughly stirring, then consider your goal. Remember the stirring mantra, stir until you are sure its done, then 1 more timed minute after that. I’ve water-thinned Tamiya and Gunze-Sanyo paints as well, but I’ve never been completely happy with the results I get thinning Gunze-Sanyo. Both companies offer a thinner and if I had to spray one or the other’s product, tomorrow, I’d probably experiment with those thinners.
If you’re going to thin a water or alcohol-based paint for spraying, you probably want to pour some of it into a mixing/storage jar. People will tell you to never return thinned paint to the container it came in, but I’ve done it and nothing particularly bad happened. Poly Scale seems about right for brushing right out of the bottle. Model Master Acryl glosses seem a bit thick, and their flat paint about right. Vallejo’s little 17ml squeeze bottles seem just right for brushing, out of the bottle.
For spraying or air-brushing, the paint should be thinned to the consistancy of whole milk. A little thicker and heavier than skim milk. This is a bit thin for hand brushing, which is why you should consider keep the thinned paint separate from your main supply.
Distilled or de-ionized water have the advantage of no mineral content to react to the paint. I’ll never forget the 1/72 Space Shuttle kit I decaled using tap water… which gave me rust stains on just about every marking! It shows up very well against white…. But I mostly use tap water and I can’t remember a problem in a long, long time. For spraying, if I want a thinner that will evaporate faster than water, isoproyl alcohol has always been my first choice. The 70% alcohol/30% water “rubbing alcohol” mix from the drugstore is fine. Use equal parts rubbing alcohol and water, for starters. Denatured ethyl alcohol might work as well. Experiment.
I’ve used my trusty Badger 250 Paint Sprayer for decades, with water and water/alcohol thinned water-based paints, especially Polly Scale, Works great. A number of people in my modeling club, The Silicon Valley Scale Modelers, airbrush with Gunze-Sanyo and I’ve done one airbrush paint job using it myself, with my Pasche double action cheapie ( model “H” or “V”). Thin, light, coats work great. Expect 3-5 to cover.
Mixing water based paint colors can be very rewarding. Start by stirring the paints you intend to mix. Use a plastic lid, white or translucent, as a mixing bowl/container and put a drop of each ingredient on it. Use a fine paint brush to bring a bit of each color together in the middle. When you think you have the proportions, roughtly, try mixing by drops, to see how it’ll go. Always put in less of the strong color, more of a mild color. Less black, blue, and red, More white, gray and pink,
Now paint it onto something, using a good brush: the outside of the jar lid, a parts tree, the inside of some part, a light-colored ‘utility’ model kept for the purpose. Just like oils, the water/alcohol based stuff changes color as it dries. That’s why I started painting well stirred paint onto the lid of its container over 30 years ago. Its a happy habit.
I’ve sucessfully used Tamiya gloss Red to mix with Polly Scale (railroad) Utility Orange, and made minor mixes of Gunze-Sanyo, Tamiya, Polly Scale and Model Master Acryl in various combinations, but they ARE different chemistry systems and don’t mix as readily across product lines as I would like. (the Red Tamiya tended to separate from the Orange Polly Scale, if too much water was used as a thinner. I’ve mixed Vallejo and Gunze Sanyo blues, and mixed that with Polly Scale white, and got more or less what I wanted. Nothing has turned a wierd color or started smoking, but I wouldn’t mix half of one brand with half of another- if the proportions are that large, I use the same brand for both parts. Although RPM makes both Testor’s Model Master Acryl AND Polly Scale, they are different in formula and not all that happy with each other, in more or less equal proportions.
You can super-dilute water/alcohol based paint and use it as a wash, just like oil-based paint. The increased surface tension of the water means it doesn’t spread as easily as oil-based paint, and tends to bead up. Use a tiny hint of dish detergent, or watch it and brush it out as it dries. I’ve had very nice results using metalic and/or black washes over brown airplane engine exhausts, dark color washes to suggest depth. Unlike oil paint, when its dry, it won’t come up again, so you can’t do the “sludge” wash to and then clean off the excess for panel lines and the like. On the other hand, with water based paint, you can just blot it up with a damp tissue, paper towel, sponge, etc, if you don’t like how it looks.