They do sell a topcoat... I never tried it. But I use Magnet Paints Chassis black and had painted over it many times. Works better and cheaper than por 15, sold at carquest/napa.

68 Road Runner, 69 Belvedere, 71 Challenger Vert
340 barracuda, 01 Ram CTD, 95 Ram, 04 Ram, 85 Daytona turbo Z
66 GTO, 06 Magnum RT AWD. 07 Ram CTD, 07 Ram