Roof Putty

All metal, metal-to-metal, and metal fusion are aluminum filled polyester products.
Regular Bondo is talc filled polyester.
Polyester by its very nature is NOT waterproof. It is water resistant.
But over time it will wick water.This is the reason they use a gel coat on boats.
I found "metal fusion" at pepboys for $18 a quart. all the others are $25+ a quart.
Andrew