If a good painter puts two full wet coats of clear on something, then he should be able to sand it flat with 600-800 with very little risk of breaking through, except perhaps on an edge or corner, which are usually avoided by a skilled prepper to prevent problems. So if you have broken through easily without eliminating orange peel, there was probably nowhere near enough clear to begin with. In a case like that, a scuff pad might be called for since anything else is going to pretty much sand through the finish, from the sound of it.