I have a 12-year-old Discovery, so naturally the undercarriage is showing some surface rust. I was wondering what the best thing is to do about it.

I bought some kind of rubberized underbody spray that supposivley stops/inhibits rust but i'm not sure if it's okay to put that OVER existing rust.

what do you members think?

 

Well the only way to truly stop the existing rust is to sand/grind/sand blastit down to bare metal, rust is like cancer, once it starts the only way to stop it is toremove it.
I would clean it up good and then spray over it, it will at least slow it down.

 

Wire brush it to get the chunks off and spray with a rust converter paint once a year. Personally I wouldn't use the thick undercoating spray for fear of trapping moisture in.

 

About 7 years late in trying to treat the rust.
When and if any breaks thru have it cut out, otherwise what you see can't be removed or stopped at this point, it has progressed too far.

 

Surface rust can be dealt with. I had forgot about this stuff, it works great: http://www.autobodytoolmart.com/pm-1...ied-wrk37.aspx

Hit the rust with a wire brush, pressure wash it real good, scrub in the above rust remover with a stiff toothbrush, wait an hour, pressure wash and wire brush it again and then hit it with a rust converter paint after it dries. That'swhat I found works best.