For a complete refinish, I have use rustoleum automotive primer. It is a very matte grey, but if you use some 0000 steel wool on it, very lightly after 3-4 weeks of cure time, it looks a lot like a blued gun. I just touch it up where needed every year. Make sure you let any spray on paint cure for as long as possible. And make sure it is completely degreased to start with.