yea a bodyshop will deffo take it back to bare metal, they'll also rub down your filler to get it perfect, unfortunately they'll charge you quite a lot for this..............the advantage of the spray can now is, it'll protect the car until it gets sprayed.......against rust !
you're deffo doing the right thing with the rattle cans..........i found it really hard to hide filler, i always used to see mounds and hills, even when i thought it was dead flat.
whether or not the rust will come back on the underside of the car is hard to tell, because it deffo came back on the Ford Cortinas, usually within 10 months, even if you're really careful, back then a respray only cost 250 quid..............amazing isn't it