I'VE always heard and been told/ taught to sand down to bare metal, do the body work, etch primer that, sealer, paint.
but here lately i hear more and more people say to etch prime then filler.... and this is from people who's opinions i respect and value. not some schnook like me.
i bought a gallon of K36 the other day, the guy i got it from said he didn't use it any more, that now days, he uses evercoat on bare metal, then uses that icing stuff over that, then an epoxy primer, then paint....
but i don't think i can hardly bring myself to lay down anything over just the minutest little smidgen of filler on anything but bare metal.