There is primer and paint available for plastic but it would cost you almost as much as new fenders, ask at an auto paint supply store. As you mentioned, you would have to strip the bike completely to paint the frame. Use aircraft paint stripper, check all welds and stress areas for cracks, too. I would use epoxy, or better yet, get a paint shop to powder coat it for you, and wait 'till they're spraying something else w/ the color you want so you can get a "batch" price on it.