Mask areas you don't want to paint. Sand down areas to be painted with 180 grit sand paper until it is straight and you can no longer feel the ridge at the painted/bare plastic line. Once that is done blow off with compressed air or wipe with a paper towel. Clean with wax and grease remover. If you can find plastic primer from the same company as the paint that will work best, if not use any plastic primer of your choice. Give two coats of primer, waiting 5-10 minutes in between and let dry a half hour after the second coat. If there is chunks on the surface sand off with 180 or a red scuff pad. Give 2-3 coats of the paint, 5-10 minutes in between, half hour to an hour to dry. Lightly sand with 400 grit. Un mask a few inches around the painted area and clear coat it, 2-3 coats, 5-10 minutes, let dry over night or even the day after that.
Also, try to blend the clear from the newly painted area to the clear that's already there, sorta feather it on to the old stuff, lighten up your trigger finger as you go out to the old paint It's hard to explain and even harder to do, but you don't want to end up with an obvious line between the new and old paint.