Advice needed

What did you thin it with, what pressure were you spraying at, and how far from the surface of the model? I use a lot of Tamiya acrylics and haven't had that problem unless the paint was too thin, the pressure was too high or I was way too close to the surface. I thin my paint about 3:1 (paint:thinner) with either 91% isopropyl alcohol or Tamiya thinner (alcohol will cause the finish to be flat). I spray at about 15 psi or less and from about 4" from the surface of the model.

The 200 is a single action brush, so you may have too much paint coming out. Try running the adjusting screw in until you can barely see the paint. If you are getting full coverage in a single pass you are giving it too much paint. It should take several passes to get full coverage.

QMeister is undoubtedly right in that there was some mold release agent on the surface, and that will cause a lot of problems with acrylics. I always clean mine with alcohol before painting and then blow it dry with my compressor. I use alcohol simply because it dries quicker than water.

When acrylics first came out, they had a nasty habit of not adhering to surfaces, just puddling up like rain water on a waxed surface. And that was straight out of the bottle, applied with a brush. However, I discovered that if I primed the surface or lightly sanded it with 600 or 800 sandpaper it covered nicely (got a bad taste for the stuff and thats why I still use enamels 99% of the time)

Acrylics have gotten much better in that respect, so I would suspect that your problem lies in another direction. The first thing that comes to mind is the surface of the plastic is greasy. When the model is molded, a mold release compound is applied to the mold (its a form of grease - I use Pam when I'm molding a part). Right before I start assembly, I wash the parts on the spue using warm soapy water and then rinse and dry them. When the model has reached the painting stage, it get washed again to get any sanding dust and any remaining release compound off the model (I use a soft bristled toothbrush to clean out any of the panel lines or crevices). I then shake to model and use a blow gun with about 40 lbs psi to remove any excess water that remains. (you'd be amazed at how much water remains in various hidden places if you let it air dry overnight, just waiting for that opportune time to pop out and ruin your half finished paint job). Once I'm sure that all the water is out of the model, I start painting.

After going through all of that, If you still have the same problems, you might try lowering the surface tension of the paint. I have heard of people adding a drop or two of dish washing liquid to their paint mixture to do that and I had a friend that was a photographer who used a product called Photo Flo (got it at a camera shop) to do the same thing for him.

Hope this helps a little.

how well did you thin it?? did it appear to run?? sounds like you over thinned it. when I've overthinned, it give the piece a tinted look in the color that I airbrushed it & the paint kind of builds up in the corners.