I never put one on over metal, only wood. But the epoxy "dries" (cures) clear enough that fiberglass sheeting becomes invisible and the wood below shows through - even mistakes in the woodwork.
On second thought, metal screws in the wood show through clearly enough that any marring of the screw heads will show through plainly, so I think it should work over metal as well.
I would practice on sections of scrap first until I had the technique down. Otherwise a soaking in acetone is necessary to remove cured epoxy. Uncured epoxy (the standard two-part type) can be removed with plain white vinegar, since the epoxy acts as a base. It's quite a money saver if you have to do lot of work around the stuff. Takes it off your hands, too, and not as flammable as acetone.
Hope this helps.
BT