Generally, I prefer to leave the E coat on the inside of the panel alone, and DA (sand w/ orbital sander) the outside surface, and completely remove the E coat that will see welding, perhaps bodywork, then etch prime, prime, block, paint,......if the panel is a trunklid, door or hood, then I remove the E coat finish from both sides, E coat is the poormans "powdercoat", it's merely an economical barrier to prevent tne panel from rusting from the manufacturer,, if it has to be maintained in inventory..........