[niceta]

I am no expert and don't claim to be but I was told by an ex body man of 15 years to use "all metal" or a fiberglass type filler for covering my but welds for a couple reasons.
1) strenthens the welded area
2) does not (less prone to) absorb moisture. In case there are small pinholes missed by welding moisture can enter from backside
3) can "help" prevent a pitted/previously surface rusted area from further oxidation/rusting by sealing it from moisture/air.

This being said if I were in buisiness of repairing cars for a living I would use All metal sparingly as it is very expensive. It does apply very nicely and sands easily as well if sanded before it cures fully. Better have some good quality sandpaper though.