Hi Muratti: You are using an airbrush to paint? If so you could also try using blue-tac (the sticky blue stuff you can buy for sticking poster to your wall). You can use it much the same way as the silly putty. If you want less defined softer edge to splotchy pattern, try cutting out irregular patterns of paper or cardboard and sticking them to the model with the silly putty/blue tac, having the ball of putty etc underneath the center of the mask will keep it from being directly against the plastic. Then spray away. Raising it off the model even just a bit tends to soften up the edges, so the pattern looks a little more splotchy. Then peel off the masks, and you can use them again on both of the kits you are building.

On the whole I would persevere with getting the best from the airbrush, change nozzle & needle for a finer one & spray freehand.