have you tried the hairspray technique, if you haven't let me explain, first paint rust, aluminum or what ever you want, then spray on regular hair spray, then paint the color it should be, the hair spray will protect the rust or aluminum from the next step, the next step can be done many ways, the easiest way is take an old paint brush soak it in the thinner for the paint you used, then run the paint off, the other easy way to do it is to take a tooth pick and cut it at steep angle the spray or wipe on some water wait a few seconds the take the toothpick and start scratching away at the paint, to explain the method in a simply tutorial watch scalemodelmedic do it

 Hairspray Technique