look at buckroseau's threads on IH8MUD. He lives in Minnesota and DDs a restored FJ40. He sprayed the tub with what he calls SEM sauce and it's survived perfectly for years, and he also lives on a dirt road so rock spray is a reality for him as well. He has a couple threads that are stickied in the FJ45 section where he uses the same stuff. If I ever to a resto I think that's what I'm going to use.
FWIW, I think there are newer, better products out there.
I haven't driven my 60 which I lined with Monstaliner on the entire underside, for exactly this purpose.
My second choice would have been the SEM sauce.
Dan