Acid etching primer is great for bare metal. It uses an acid additive that microscopically eats into the metal surface and gives a very tough bond between paint and steel. Only a light coat is needed, followed by a sanding coat of regular primer. Regular primer is used to build the surface so block sanding will result in a perfect base for final paint. It acts as an overall filler to help level the surface and is designed to bond well to underlying surfaces and overlying coats of paint. It isn't intended to be a final finish so will absorb water, degrade and chalk of if not protected. Cars with a nostalgia 'suede' or primer finish should be clear coated with flat urethane clear coat.