I'll warn you up-front - I'm pretty much an amature.
This is the only method I've come up with - but I'm all ears if anyone has a better technique.
I mostly do hand painting of canopies with acrylic paints. When I goof up, I take a toothpick and chisel it to a sharp, short point. If you make it long and thin it doesn't work. It has to be stubby but sharp (am I making sense ?) then I gently push the paint off the surface. I don't think this would work with a dried enamel paint. If you push to hard or rub too much it can leave little scratches in the plastic. I hit them with Future and that usually helps reduce the visibility of the scratches.
My 1/72 avenger "greenhouse" came out looking pretty good - I liked it.