I'm limited in my area (Buffalo, New York) to Tamiya, Model Master (Testor's) and Polly Scale acrylics. I've found that thinning Tamiya acrylics with rubbing alcohol (from the drugstore/chemist) works just as well as their own thinner (20A) and a lot less expensive. Also, I've thinned Testor's with their own thinner with great success and also with household ammonia based window cleaner (Windex here in the US). If you're spraying a small amount (i.e. an airbrush cupful) I use no more than 1 DROP ONLY of the thinner/Windex..Any more than that and I get what happened to you...runs, specks, etc. With Polly Scale, I strictly use Windex, again, no more than 1 drop per cupful.
I have a Badger 155 Anthem double-action airbrush which I typically run all the above paints through at 12 to 15 psi. I think that if you apply a good primer coat, don't over thin your acrylic paints and be patient and make multiple misting passes, you will get nice smooth coverage with yellows, reds and whites...those typical colors which make me wonder why I do this for fun sometimes!
John Z