I was watching modeling channel on you tube ( TREK WORKS ) and saw a guy using cheap waterbased craft paints on plastic models with an AIRBRUSH . He thinned it with water and ran it through an automotive paint strainer and it worked well. I just tried this on aHO caboose. I used the very cheap Walmart red thinned with a mix of water/ Windex used a strainer and came out good after some time shot some micro-scale semigloss . After a day added some water slide decals then a dull coat and it all worked fine. I will be using this more in the future. This is not a replacement for RR colors paints but for custom colors is nice. Just wanted to share.
The cheap paints work well for washes too. I used a lot of the less than a dollar 4 ounce bottles of craft paint form Michael's. Plus if you watch the sales, they have a 50% off coupons now and again. Buzz.
I've airbrushing craft paint (mostly "Delta Ceramcoat") for some years. I mostly use it for weathering but have sprayed it on structures also. I have found that you need a good primer coat so the plastic has a decent tooth. Rustoleum automotive primer in a spray can has worked for me. Without a good primer the thinned paint can bead up on a smooth plastic surface. It will also peel off - you need not ask how I know this! I mix it a little bit thicker than the "like 2-percent milk" guideline for spraying a color coat (as opposed to a weathering coat). I've been using distilled water for thinner but now am trying Testor's airbrush thinner. What I really like about craft paint is that when it's maybe a buck-fifty for a 4 ounce bottle you can experiment and throw away what didn't work without getting a second mortgage. Craft paint pigments are not as finely ground so it's harder to apply a really thin coat. I do prefer Model Master or Modelflex for my locomotives and rolling stock, but here's a Athearn S-12 switcher that I painted and weathered with craft paint some years back. The detail did get filled in a bit more than what happens with model paint. This locomotive is Ceramcoat "Hunter Green", airbrush weathering is "Autumn Brown" and a mix of Burnt Umber and white. The rust streaks and spots are drybrushed Autumn Brown. George V.