600 wet is plenty enough for mechanical adhesion. It will feel smooth but the basecoat will adhere. If you shoot it over 220, you will see every sand scratch in the primer. Personally I would finish the primer with 400 wet, then use a reduced coat of epoxy primer as a sealer before basecoat. Plus the benefit of epoxy is that it will tie the coats together chemically, as well as a uniform colored ground coat.