Use layers of Metal2Metal first to smooth the weld and build up the surface even. Then use regular filler over that as a final surfacer. Build the layers and sand each, taking your time and keeping the layers thin.
I used that on the seams of my quarters when I welded mine on. After some research and posts, I was led to believe this was stronger and better than regular filler for the thicker application on the seam.
All-metal or Metal2Metal do not absorb moisture like the bondo type body fillers. As Laurie mentioned, using the metalized filler first then covering it with a thin layer of bondo seems to be the best ticket. The metalized fillers are a bit trickier to beat into submission.