Snowseeker rocks! - great photos and explanation. I grew up in Michigan and we would mix 90wt with gasoline and spray it in the doors of our cars. The gasoline (could use turpentine for a better smell) thins the gear oil and allows the gear oil to soak into the cracks, and then evaporates. We also used to spray the underside of the vehicle with oil.

If you paint or put spray tar on rusty surfaces, the paint coating will ultimately fail and make the rust grow worse by trapping salt/water/dirt against the steel. (Some POR type coatings do seem to work well...but I would check them often.) Anyway - for chassis rust and for welded/folded seams, spray them full of ATF, used motor oil, gear oil or whatever.

 

When I lived in NH......WE used Chainsaw Bar-Oil.
This stuff is "oil" and it is sticky. It is cheap and easy to spray on each year.

Now I live in Montana.... Salt not allowed!
No rust....

 

I have the same issue as you do Kyle, but I only have front fenderwells to worry about. The worst part of mine is the rocker right behind the fender. My tires kick up gravel from the lovely limestone roads and has sand blasted the paint off. Luckily it hasn't rusted because of the constant sand blasting effect, it is just bare metal.

My plan is to tackle this project this winter when I have more time (and money). I am going to completely prep the surface and plan on using a paint matched rubberized coating (Line-X or similar) from the trim line down. I also have significant paint chips on the hood and fenders, a dent on the roof that is starting to rust, and the paint is pealing off the hood so I may just paint the whole thing.

The bed is coming off and getting sand blasted and Line-Xed. I am getting tired of driving around with a rusty bed.

 

Have you looked into Fluid Film?

 

What did you use to apply it? I'm guessing you did it in tyw grass somewhere.

 

Air compressor ......air gun with tupe that sticks in the gallon jug.
Jack up the car/truck...put on googles....(in a dirt driveway) Let it fly....

 

I have heard of Fluid Film. Is it recommened? How good is it?

 

I've just had it on my oil pan and diff. cover about a week..... stay tuned and I'll let ya know. They were already started to rust so I should know in a few more weeks. I've used it in non-automotive things and loved it..... we shall see.

 

OP I'd consider painting the inner wells with bedliner spray. I use Rustoleum bedliner in the rattle can.

I also use the used oil method under the truck and in every crevice. I mix it with kerosene to thin it a little. I spray it with a hand held industrial strength widex type sprayer. Good luck as you will wear some, but it gives a nice matte black finish on all rusty non painted parts like Snowseeker said. Let it soak in and go for a drive. I sprayed all between the body panels so it would run down into the creases of the fender and door bottoms. Avoid spraying the exhaust or your truck will smell like a GM truck for a few days. Also obviously avoid the discs but the brake components all get pretty rusty so find a way to get those. I can smell the oil just thinking about it. ahaha

 

Although your truck is a little bit newer, is it showing any signs of rust near the wheel wells since you've been spraying Rustoleum? I've been tempted to use it, but just never got around to it.

 

Are you talking about the Fluid Film? I would love to hear if it works good.

 

I just had Ziebart do the 1,2,3 on mine. They put it on a lift & power wash the ***** out of the undercarriage. Once dry with compressed air they spray an encapsulator in every nook & cranny. That dries then they hit with the rust proofing. What a day and night difference. My 08 was just starting to show signs of corrosion around the edges of all the air holes punched out in the under body and various other mounting points and other areas. The power washing blew that down to the metal. The encapsulator seals it and converts/neutralizes the oxidation to an inert black spot. Then the undercoating further seals it off. I know the owner so I was able to watch the process.
Then I had them do the inner panel sheet metal protection. It’s most likely LPS3 as it has a waxy like residue. They got into the doors, the tail-gate, the box, hood ribs, inner front fenders etc...Should last a while now. The price includes a touch-up annually & he tossed a window tint job in for free.
The biggest thing I do living in the rust belt is religiously, even when it’s snowing out, rinse it off as soon as I get home. I'm talking bent over with the hose rinsing all the inners. I have a hot/cold bib in the garage. I get the water everywhere especially the bed rails, inner fender wells, drain holes in the doors and under the door seals etc...My neighbor’s think I’m a nut case, they’re probably right. Get under your truck and get familiar with the hiding spots for dirt/salt. And look at another truck that’s already rotting. You’ll see what I'm talkin about. Then you not looking for the trouble spots when its 30* out snowing and usually dark.
I have a 95 GC Jeep that’s just now showing rust through in their typical spots. That’s 18 years of winter. Most 95s around here are already rotted & crushed. It has the electronic rust controller on it from new which I’m sure helped. I did some homework on a similar electronic sacrificial anode inhibitor. They sound good too. I might add that into the mix too. Expensive but so is a new truck.
The dirt holds the salt and moisture in so by keeping the inner structures clean it helps. Some will argue keeping it wet isn’t good either while I somewhat agree, I say better than salt.
To answer your question no one thing will keep the rust out, you need to do all the above starting with cleaning the hell out of the underside and all inner panels. Then apply the encapsulator and paint over that? Or if you can afford it do the Ziebart thing. From your pictures it looks like just surface rust and the seams are just starting to rust. Get at it now don’t wait.

 

No signs of rust at wheel wells. I have seen more of it than I like to see underneath and at the top of the fenders under the hood. I havent done the spray on my inner fenders yet. I have used it to elimanate rust from trailer tungs, work van bumpers and wheels though with excellent results. When/if my chrome parts start to pit my plan is to paint them with the rattle can liner too. It has a nice finish which is durable for a paint, but IMO I wouldn't do an actual truck bed with it. I'd go professional for that. Ive also been using the oil spray under all my trailers and it is working nicely there too. I just freshen it up occationally. It seems like it will be messy if repairs of the treated parts are needed, but I guess I can degrease it first.

 

Had to get under the truck today to check something else and after looking at what the Fluid Film is doing, I wish I had sprayed the whole underbody the day I got the truck.
Even with the surface rust started, in a weeks time it has almost eliminated the rust and looks like it is protecting what hasn't rusted yet.
In the back yard I have one of those big barrels (40-50 gal) for collecting compost with the crank handle. The bearing on it were squeaking like crazy. I tried 3in1 and WD40 and other stuff in the bearing. In less than a week it was squealing again. Fluid Film had their promo here about that time so I got one of the small spray cans. Sprayed that stuff in each bearing and haven't had a squeak since!!!! That was well over a year ago now..... good stuff. Bought a couple of big cans and now that's about gone. Gotta get me some more of that stuff......

Oh yeah...used it on all the screw heads on the RV.... you've seen how they rust after a few years.... no more rust streaks down the side of the trailer! Just a dab will do ya.....

 

I need to spray my truck with fluid film this winter.
I use it on things all the time and REALLY like it.
They have started to coat the highways in CT with a liquid brine(salt) solution in the winter and my truck is starting to have some rust on the under side.