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The same company that makes PB Blaster penetrating oil also makes a product called Surface Shield. It comes in aerosol cans, it's very inexpensive, and you can do an entire Super Duty with only three cans. It's extremely easy to apply and I did my own truck last year in less than an hour. It held up fantastic through the winter and I'm very impressed. I shot a YouTube video just to show the stuff and what it looks like when you apply it and then what it looks like after going through an entire winter.
I tried POR15 and it failed after 2 years. I then went to Eastwood and got their Chassis black and Rust Encapsulator Plus and used that on the chassis. Looked great and held up very well. The POR15 is UV sensitive and MUST be top coated, which is why it failed on me. Either product will make it look good, but won't protect it. Wool Wax is the choice for protection. Can do it yourself or have it professionally applied, the latter will last for YEARS! My 2019 was done by this place soon after the OO bought the truck, and has not been touched since, still very well protected though I am starting to see a few areas getting thinned out. It is better than Fluid Film only because it is heavier bodied and holds up better to underbody sprays and salt than FF can. Same material, just heavier so it lasts longer. If you were to have a gallon of each side by side, and put stir sticks in, the FF stick would eventually fall to the side while the WW one would stay upright indefinitely. The places I have that the WW has gone away are the leaf springs and the differential, the first just rubbed away, the second from heat where it evaporated and washed off. The underside of the cab and bed still have a thick coating as well as the frame and most of the suspension/steering. This was applied early 2020.
On my inside of my bumper I used the POR-15 Rust Preventive, two coats and then followed that with the POR-15 Top Coat. Hopefully the Top Coat is enough protection from road debris that gets kicked up to the bumper.
After painting the bumper I decided to pound out a small dent with my pneumatic hammer. The paint lasted about five minutes which was much longer than I expected. The pneumatic hammer didn't get the dent out enough, so I used my hydraulic press which did.
Whatever rust mitigation you use, be sure it's gooey and greasy, and NOT a hard, drying tar-like 'sound deadener' undercoating. Those eventually promote rust and are pure evil. Some paints can be effective for a time, but anything entombed in grease will not rust.
If you get the spray gun from woolwax it comes with a flex hose to do that. But that will require a compressor for the air and then you just get a gallon can from amazon to do that.
I use the spray gun and gallon container. I have several cars/truck/trailers I do.