If they are small pimples, this can happen if the paint dries as it leaves the gun. Keep your PSI low. 35 PSI should be ok for undercoats. It should go on smooth, but sometimes dry with pimples which are sanded out. The distance between the gun and the surface should be the same as the distance between your outstretches thumb to pinkie. Also, don't paint in the sun, and don't paint in less than 20 degrees temp outside.
It takes a lot of practice to get it right, and once you know the gun and the paint, you can make adjustments as you go along.
The Protec paints as described by Sinbad79 are very good to work with.
