I was planning on undercoating with the 3m spray cans of undercoating I had a case of on hand. First, I was going to wash with simple green or some other tsp substitute. Next, I'd spray the 3m undercoating. Lastly, I was going to consider spraying the inside of the frame with fluid film or even used oil.

Thoughts? Im not sure if any of the above plan would damage rubber components, but I don't think so. I just want something to protect the aluminum and steel frame parts. It's a brand new truck, so I'm not expecting to see any rust going in. Haven't taken delivery yet. My 04 is barely starting to rust around the rear bumper corners, on some frame scrapes, weld joints, and fender rolls.
​​​​​​

​

 

If it's rubberized, even 3M, I'd stay away. It'll eventually trap moisture and begin to rust which will accelerate with time.

Being brand new, I would consider spraying it with something like Cosmoline- it's a creeping oil based wax coating that becomes a little tacky but doesn't attract dirt (RP-342, etc). It's salt and water resistant but not to rock chipping.

Or look into Line-X undercoating. They spray a hard forming heavy duty undercoating. With the line-x, I hear that any rust would need to be removed before hand. Even though it's new, I've seen them come from the factory with small amounts of rust on the frame.

I've done the fluid film thing and while it does work, it melts in the summer, meaning it's tough to know if it's still covering the inside of the frame rails. I checked mine with a borescope and there was mountains of fluid film and dirt collected at the bottom of the frame. I'm sure it's fine, but didn't leave me with peace of mind. A bunch of different companies make internal cavity wax, which may or may not work. Spraying with cosmoline might be okay, but you have to get full coverage.

Lastly, I think that fluid film may strip cosmoline. I know for sure that it strips the factory NOX coating on GM/Chevy frames.

 

It is hard to beat FLUIDFILM, at least as far as a spray can or otherwise sprayed product is concerned. The rubberized undercoatings tend to crack and peel over time, and as SC 767 notes, the metal will start to rust underneath.

 

Waxoyl is really good stuff and easy to apply.

 

Beware that with any of the oil/wax based products, it becomes a royal pain to work on in the future.

One of the previous owners of my TJ had it coated and it is impossible to do any underbody work without creating a significant mess. It's black, sticky, and very hard to dissolve. In addition, it often catches fire inside the frame rail when I'm welding on the frame. That said it definitely worked well at preventing rust...

 

Yeah I'm a little hesitant to run with the fluid film because I don't really view it as all that permanent or reliable. My current truck has the Linex undercoating and that has been good, but does chip and eventually show a little rust after a decade. The 3m stuff is advertised as preventing rust AND I already have a case of it. I threw a RFQ out to Linex to see what the cost is these days.

 

I've use woolwax/fluid film inside and outside the frame on a couple trucks now. Can confirm, working on it is a mess of black goopy never dried stuff that gets everywhere. On the other hand, even the areas where it gets blasted off from road spray in the winter still bead water in the spring. Every fall I power wash any excessive built up dirt/grime off and reapply new fluidfilm/woolwax.

For the rust belt, I believe it to be a good trade-off. Even my brother, who is a mechanic in Wisconsin prefers a fluid filmed vehicle over a rusty one. Their shop even does fluid film as a service.

Now if you never see salt, then I probably wouldn't use it. Keeping it clean and painted would be more than adequate.

 

There is definitely salt in Colorado. My Virginia truck had nearly 0 rust before moving to the coast, where tunnel joints leaked salt water. I cleaned it religiously and ended up doing alright, but then things are on the steady decline after moving to Colorado.

 

My own biased opinion, but undercoating spray is from the devil. I recently completed a car restoration project, and some previous owner (or his mechanic) sprayed that black junk almost everywhere, inside and out. If there is a way to keep it just on the target panels, maybe, but once it gets on hardware, plumbing, and wiring, it causes problems sooner or later.