Fork legs are alloy with a coating of (I think) either paint or laquer. The corrosion on the legs is where the salt and weather has got beneath the coating. If you rub them down enought to get at the corrosion, you will have removed the paint/laquer and exposed the alloy. Hammerite/Smoothrite needs
its own primer on alloy before you apply the topcoat as it doesn't bond with alloy in the way it does with steel.