Bobbie, I have used Rusty Metal Rustoleum 7769 for light or heavy rust on steel. It is a primer so it does need topcoating after complete cure time (several days in warm, dry weather).

What I like about the RustCap is that it doesn't require a primer or rust inhibitor, just scraping off of loose rust. But I have no idea if it will continue to work with actual exposure to time and salt water. Bobbie