Well, how much rust? General procedure for extensive rust is to sandblast back to bare metal, then immediately (do not wait) seal with epoxy. Old schoolers like what's called coal-tar epoxy. For spot repairs on a less affected keel, you could sand or grind away the rust, maybe apply rust converter (phosphoric acid), then seal with epoxy. After doing either of the above, fair the keel using epoxy thickened with microballoons and sand to taste. Be prepared to do it again the next time you haul out, but if you did a good job the first time, it won't be as bad.