I bought a used 2011 Navigator over a year ago. Really like it. I want to keep for a long time, and have started my own DIY maintenance program on it. While I was lying on my back underneath the truck I noticed some light rust on the frame rails. I live in PA and with the winters and salt I figured it wasn’t going to get better, and decided to do something about it. Truck had 60K miles on it when I did this rust treatment.

Initial Observations: Frame rails had light surface rust on almost every weld. I think when they construct the frames, the welding process leaves a residue in the local area of the weld. They probably don’t clean or prep the weld for painting. The result is lower adhesion of the paint on the welded and surrounding area. Over time and exposure to the elements the paint weakens and you get rust. The three crossmembers seem the worst. Below are two examples of what my frame looked like before.
When I was looking for rust, I dropped the spare tire. The back of the truck in the spare tire area also had some rust in a few spots on the body underside. I used the same treatment on the sheetmetal that I did on the frame. I didn't take any pictures while I was doing this do I downloaded some example off internet, but the rust is very similar to this vehicle below.





Treatment Process: Got a drill mounted wire brush and a hand held wire brush. Scrubbed the loose rust from all areas. You don’t need to get it all off, but more is better. Wiped all areas with mineral spirits to prep for rust conversion
Loctite Rust Neutralizer. This is one of several products that chemically converts rust to a hard plastic like substance, that you can sand and paint. Eastwood and several companies make it. The Loctite brand works good. Apply to rusted areas with an old (or cheap) paintbrush. Can says you should use between 50 and 90 deg F. Wait 30 minutes to dry, and apply a second coat. You don’t have to be neat about this like paint, you’re just getting the chemical on the rust





I waited a day for the Rust Neutralizer to cure. It will change the rust to a black-purplish color. As long as the frame is still clean you don’t need to prep before paint. I had driven mine and so I wiped down any dirty areas again with mineral spirits.
Paint: I had some spray cans of Eastwood Chassis Black paint from several years ago. This is a good petroleum based paint for underneath cars. Lots of solids in it and tough. Sprayed two coasts over all the rust treated areas and any other areas that looked like paint needed touching up. Used a hand held piece of cardboard to mask & protect any wiring or devices from overspray. I did this inside my garage so wind didn’t blow any overspray on my exterior paint finish. I wore a respirator that I get from work.

Again this photo was on the internet, not from my project. See how every frame weld in the photo below has rusted. I think this happens frequently as part of Ford's frame manufacturing process




There you have it. Entire frame and underside of truck looking brand new for about 5-10 bucks. Probably took about 2-3 hours over a weekend and waiting for paint to dry. I think this is worth doing for you DIY types. I feel like I almost reset the clock on underbody rust on my truck. If you live in a winter/salty area, an extra five years of life on rust progress on the frame could extend the life of your vehicle until something else goes.