The actual product I used for the keel was Jasco 'Prep & Primer.' This product converts the rust from ferrous iron to iron phosphate. You can apply it with a brush to new metal or to rusted metal.
Another product that works real well is Loctite 'Extend.' Use this product on lightly rusted iron. I have used this on exposed metal on my trailer and other rusted objects that are open to the environment. It does not work on shiny metal...only on rusted metal. I have only seen this product in spray cans, but I understand it is available in a brush-on liquid as well.
A third product I've heard of is called 'Rust Mort.' I've never seen this stuff, but I understand it works like the others do.
I have no direct experience with the Rustoleum product, but it sounds like the same sort of primer as those above. I'm afraid my previous post sounded as if I had used the Rustoleum product. Sorry about that.
After using the Jasco product on the keel, I used fairing compound to fill and fair the keel. Following that, I laid a piece of fiberglass cloth along the leading and bottom edges of the keel, wetted out with epoxy. I then put several coats of 2-part epoxy (I used West Systems) on the keel, followed by my antifouling.
I have observed no rust yet on the keel, nor has any rust appeared on the trailer parts that I coated with the 'Extend.' I got the idea to use these products from a video that Catalina produced on refinishing the swing keel of the C22. The gentleman who demonstrates the process on the video sails his boat in the Great Salt Lake...many time saltier than the ocean. So I figured if this stuff worked on his keel, it will work on mine.
Hope this helps.