I use Evercoat Quantum1 and it specifically says you can apply direct to metal and it will survive the 500 hour salt water test. It's expensive mud, but it sands like butter and doesn't seem to shrink back hardly at all. Q1 is a hybrid polyester/epoxy filler, so that makes it more unique than just a standard polyester filler. Plus it also works well as a finish putty. Way better than Rage in my opinion. Epoxy prime before filler is cheap insurance, but I went direct to metal on the last job I did and so far it's held up just fine.