Yea, I've given that a lot of thought. Wish Toy had designed stuff like they do now. Anyway, what I did is spray the heck out of the underside with "Rubberized Under Coat". Clean everything first of course. That does help, but may need to touch up once a year or so.
I think you would get the best results if you use a high film build, soft material. Rubberized Undercoating would do it, or truck bed liner would be even better (better adhesion). Soft coatings will not chip, debris will "bounce" off the surface.
To do a trial, apply the coating to a test panel. Then sand blast it. I think that will give you a good idea of the level of protection.
With any kind of rubberized coating, I am concerned about water getting trapped under the coating and causing hidden rust. Also the PO had some of this stuff there and it is peeling away like crazy. I plan on scraping it out and want to put something more permanent. I think maybe Chassis saver or Rust encapulator might do the trick.
What ever you do, make sure that what you cover has been treated; all rust all surfaces. follow all directions for chemicals used.
I have used POR on several applications and it works well, but it takes a prep with an acid zinc wash. Once applied it needs to be washed off. then DRIED.
Flash rust is not a problem. Then your can apply the POR. POR must not come into contact with any moisture.
I've used eastwood products for years with no compliants, I prefer to used the encapsulator for hard to reach areas, specifically areas that do not come into contact with road debris.
then apply a QUALITY rubberized coating. Follow all directions and allow ample dry time.
I believe I would use the POR-15 on surfaces that are subject to abrasive or stone chipping environments. It just takes a lickin' and keeps on tickin' without failure. If you have any questions give me a shout. Good Luck keep us informed. Kenn
Try zero rust. It comes in a spray can or in qts. Can be sprayed or brushed. Thick solids. Be sure to get the pre step powder to clean first. Reasonably priced and easy to apply. Comes in colors too. I am using it on parts of the frame and body and it seems to be pretty good stuff. I chose this over por 15 because it is safer to spray, and easier to apply. Time will tell how well it holds up. I like the way it looks. It has a satin finish. And no I don't sell the stuff.
This is what I do. But cruisersolutions has been using line x under tubs and fenders for a while now. The spray on bedliner from a rattle can chips really easy. I have line x on my bumpers and it's very rugged. If I was planning on driving mine in the salt again, I would do line x. Rust encapsulator is good stuff, but I certainly wouldn't expect it to deflect road debris for very long. it certainly didn't on my plow truck.
I 2nd the zerorust, used this on my rebuild and couldn't of asked for a better sealer, did my tub, frame and interior in it. Dries nice and hard. Just make sure to clean well and use tri sodium phosphate when you clean the metal.......