a surfacer sands easier than a sealer .... if you use epoxy to start, you don't need a sealer, but a surfacer is a good idea because epoxy is prone to be a bit textured and very hard sanding .... a 'sealer' [primer] is just that, to seal the existing surface before other treatments, both to keep any under lying color [like bondo] from showing thru and to allow further treatments to have even coverage - a 'surfacer' [primer] is more for filling minor scratches/roughness so it can be more easily sanded smooth
Bill
