a Green 3M pad works great for prep work, to give the paint a bit of a tooth to bite to. Even a good replacement for many different grits of sand paper. Etching primer is meant for bare metal, but still a great product.. -I've done a LOT of painting in my day.
If you want a high gloss shine, or even a flawless paint job, wet sand with 1000-1500gt between coats. Even between primer coats. Sanding is a VERY important step for a good paint job. Clear coat would mainly be used for a high gloss shine after the base coat is down and cured.