The thing is that if you want to paint your lexan with anything else than lexan paint, go right ahead. But paint a primer layer with lexan paint first so the other paint sticks to the lexan. If it is on the outside you want to paint. Prime with a lexan paint relative to the paint that goes over. If it is on the inside you want to paint, use a layer of tamiya ps55 first so it sticks.
Personally I lean towards outside paint on scalers, looks more realistic.
I've used the same method. I think this works best, and there is no extra prep work required. I paint the inside and outside with a Tamiya PS paint that I don't mind showing through when it scratches, then go to town with my color of choice. I used this technique to get a nice matte black finish on an 1/8 scale buggy. PS black on inside, PS black on outside to act as a primer (mask windows inside and out. I just trim out the protective film with a razor blade for masking on the outside), then testors flat black model spray. The body has taken much abuse on top of the 1/8 scale, including skidding upside down across pavement at high speed. No chipping or flaking at all and scratches are pretty minimal as well.