You know, AutoZone and most of the other "auto parts" shops have rattle can rubberized undercoating for around $5 a can...I've coated the underside of my X, and so far, haven't had a single bit of rust come through (and that's going from upstate NY with the road salt to San Diego and sea salt, banging around on AZ trails and Rubicon). That stuff's a lifesaver...if you're having rust show through get some Naval Jelly (also at AutoZone, and cheap) and rub out some of the rust, allow it to dry, then coat with the rubberized undercoating. Stops rust pretty well. Just be careful around some of the moving parts (tape off the mating surface of the propeller shaft and rear diff, avoid plugs for diff oil, etc) and you're golden.