I think a 50-50 mixture is a pretty safe ratio to begin with, using their thinner, and after ensuring that you've washed the plastic parts to remove any release agents. I set my airbrush at around 25-28 psi and this combination works well for me. Lately, I've been adding a drop of acrylic retarder to my paint mixture and I find that this ensures nice smooth coverage. Tamiya sells their own retarder but I just picked up a generic bottle at a local art supply store.
If you're using a double action airbrush, alternate back and forth between applying paint and blowing clear air as this enhances drying time and eliminates any chance of a run. If your paint appears wet, just blow clear air on it until it loses that wet sheen and then start applying paint again. This works really well and is a tip that was passed on to me by Tankbear, who is also a member here.
Using this technique, I'll spray a second colour almost immediately after the first, unless I have to mask. When masking, I'll wait a couple of hours first, just to ensure that the paint is fully dry. Others may disagree, but just to be safe, I usually seal any paint before masking for a second coat. That's always worked well for me until my present S.E.5a build.
Cheers,
Chris
