Airbrushing problems

Ok, I had similar problems when I first started airbrushing. I use Tamiya Acrylics almost exclusively and I only thin the paint with either Tamiya Thinner X-20 or 90% isopropyl alchohol. I use a double action Badger 150 bottom feed, and I get my best consistency by thinning my paint on a 1:1 ratio (paint to thinner) at a psi of between 25-30. Keep an adequate distance for basecoating and do not exceed 30 psi in my opinion when basecoating. Always practice on a piece of styrne or plastic. I find using cardboard will only absorb the paint and not give a true color or consistency. If you are doing camo patterns you can thin the paint a little more and spsray at a higher psi, no more then 35 (again in my opinion.) Thosecircles your getting is because your not moving the airbrush at a steady consistent rate. To thinned paint sprayed atthe same location point to long will cause it to build up at the edges. Also make sure you mix your paints enough before adding the thinner and then mix again. You might not be mixing your paint enough, and that could be causing it to appear to dark. Priming a model in white or light grey will give a much better color tone to your basecoat. As for the little dots, it is definately from one of two reasons. One, Acrylics dry fast and you have to make sure your tip does not have dried paint on it. If it does it will case splatter or stippling on the paint. Just dip a cotton swab in thinner and dap the end of the airbrush. another reason for the dots could be that your paint is to think, and comes out of the airbrush in a sort of clumpish fashion. In a nut shell, make sure the paint is thin enough for a nice consistent spray, make sure you mix the paint, make sure the tip of the airbrush does not have dried paint on it, move the airbrush in a steady fashion (not staying in one location point for to long, gradual spraying and building up of the basecaot is better then giving it one big coat), and making sure your not to close to the model, and spraying at a strong enough psi. This is all my opinion and what I do when airbrushing. Hope it helps.