I'm not at all sure if Body Filler would right for the application your talking about. Is this a cosmetic or structural defect?
I have used Epoxy Putty like KBS NuMetal for something similar to what your describing.
Actually the stuff has proven invaluable for a couple of fixes both for cars and around the house.
You just follow the directions for prep and combining equal amounts of each "stick" together relative to the size of the repair you need to make.
Once applied you wet your fingers and smooth the material out over the repair. The smoother you make it the less sanding you'll need to do afterwards. Use water and a rag to clean up any residue on the work piece.
Once cured it can be sanded and painted or just left as is and forms an extremely hard and durable repair.
The problem I see with body filler (albeit a metal based one) is that they aren't really good at taking a pounding. That is to say they work great for sheet metal panels on cars but I've never used them for making repairs on other items.
If you do decide to go that route, you'll need to clean then sand down the repair area with 180 grit sandpaper to give the metal some "bite". Apply the filler according to directions (press it into the metal and use multiple light coats to "build" up the repair). Once your done you'll need to sand down the repair to get it flush, clean again then apply some paint.
Hope this helps
Thoughts and comments expressed by me are mine based on my own experience and research and shared here freely. I am not a professional nor make any claim to be as such