I have used a two part metal bonding agent with great success. It CAN'T be used on structural metal though. It works great on a small patch that will not be put under much stress. It is superior to welding for this application because there is no warping or other heat damage. If it is used correctly you can get by with less filler on the seam. I have seen a patch hold up to a direct hit in a collision. It is a perfect fix for a small rust hole in a quarter, fender, door ect.
Jim

Salmon! LOL!
I'm slow, must be election hangover. Good to agree.