To your point IDIoit, all the reading I have done says weld it grind it down and then epoxy it for that very reason you mention. The filler won't seal out the moisture. So I'll get the metal back in close and epoxy it all and then I'll sand the damn body lines into shape if I have to I guess... or throw my hands up and say screw it because as you put it, I can't see it while I'm driving. That means that just the hood has to be straight right?
theres 10 million ways to skin a cat.
if you read about bondo, its supposed to be placed on steel.
if you place it over a primer, either epoxy or poly, you have a barrier there which will sometimes pull away from eachother, depending on the grade of the filler and the grade of the primer.
you cannot simply call filler, filler.
there are 3 types of filler i use on body work.
a fiberbond type of filler, which does not let moisture in.
a normal type of bondo, which will absorb moisture
and a glaze. ( to fine tune your prep work)
i was taught by a top notch painter that all of your major filler work should be done before any type of primer.
this method just stuck with me.
again, the only way to learn is to do it and fail.
we make our best decisions of the failures we have committed and seen.
no way is a wrong way, until you hit a bump and a chunk of bondo falls out