Best thing you can do is get a little air compressor and a scraper and clean the underside of the deck every 2 or 3 mowings. (This seriously only takes 3 or 4 minutes with a compressor.) We got 25 years out of the first deck on the Cub doing this, (a large crack on the front did it in, not rust), and now have 9 years on the one on the Craftsman and it still looks excellent underneath. It's when you let the grass pile up under it storing all that acid and moisture contunuously is what makes the deck rust through. There are few if any paints or coatings that are going to stand up to every nick and ding the underside of a deck it going to get- normal cleaning will make the most difference with deck life.
-Fordlords-