So as I sit here gathering parts for my 66 F100 (351w) Flareside it dawns on me...the cheapest way to fix rust is to prevent it. I want to put a undercoat on while I have the cab removed from the frame. My question here is what do you use. I'm leaning towards a rubberized "Body Schutz" from 3M. Anyone use it? How has it held up?

 

I am going to be the naysayer here.
These trucks were not made like vehicles today, with an eye, if only a minimal one to water entry and rust prevention.
I have completely disassembled 2 '66 cabs, to include the doors and front clip for restoration. I have also had to scrap several more for serious rust. These trucks have so many, many - did I say many? - places for water to enter that it is, in my opinion, not possible to significantly reduce its entry.
Some of them are so large and are needed for drainage from areas higher up that to plug them you will end up retaining salt water in the hidden areas.
Some you cannot get to as well.
Another place is the doors. They have drain holes in the bottom, and you don't want to plug them.

In my own humble opinion, the best way is to keep out the salt rust is to keep the truck out of the salt.

I have a three-rain rule. It has to rain hard three times after a salt application before I will put my truck back on the road.

Just my thoughts.

John

 

I guess I am not only concerned about slowing rust but "prettying" up the bottom of the cab and frame.

I will say this being my first restoration project it is quite addicting.

 

Oh yeah on the addicting part!
-which is a large part of the reason I keep my truck out of the salt, because I want to keep it and not have to repair any cancer rust.

As for the underside, just paint it the same as the body color. All the trucks I have worked on came that way, but I understand that some left the factory with just primer on the underside.

Have fun!

John

 

I agree. A couple good thorough coats of 2 stage epoxy primer to start with that you never sand through, sand able layers on top of that to work from and smooth and then paint on top of that. The epoxy primer doesn't absorb moisture and can be left as is without topcoating, too, even on the underside.


I've found that some of the places most frequently rusted is where they applied body putty in some of the nooks and crannies at the factory to fill small voids and openings, often underneath. The putty itself can hold moisture and trap dirt and crud.


I will say some undercoating can be good for reducing road noise, though. But, I'd make sure there is a good layer underneath it as a base.

 

I did mine 3 sections: rear with the bed off, cab while I was doing extensive rust repair, then front during engine bay and front end work.

I have no idea how well its held up as I dont drive it enough to call it a real world test. What they're saying about water leaking and drainage is true. I sealed off everything I possibly could during the whole rebuild process. Weld it up, silicone seal joints and welds, primer and undercoat. I even took out my air entry grill to the front cowl (opening in front of front window). I havent put and floor liner or carpet in yet, just so I can monitor water entry. Sure enough I have a small leak over the front window and one at the wind window, also the passenger door seal. Like previously stated the whole truck takes in water and drains it, so I did my best to coat anywhere I could and leave drains open (like cowl drains, under the truck at bottom of air cowl). The new cab mounts came with drain holes/notches in them, I sealed those in attempt to prevent anything getting up in there anyway (and of course coated them on the inside as well).

So after letting it sit outside for a good amount of time in the rain:
- water that gets to the floor pan almost always runs to the bolt that goes through the pan and cab mount to truck frame, mine is siliconed shut so I clean what little gets there up and will repair leaks, but you can see how it would drain right through that hole, down the bolt, and in to the cab mount, then hopefully out the drain slots the manufacturer of the new mounts put in it.
- Even though i sealed the front cowl, water still comes in through the tiny drain holes in the windshield wiper and on down the cowl....except somehow I wound up with a puddle of trapped water right behind my passenger side door curvy section that tends to rust right by where the bolts for the door which I though was completely sealed off from the cowl and its drain areas after my rebuild. It was coated, but now has its own good size drain hole. (I park leaning toward the passenger side so thats where water would tend to drain, as apparently did my grandpa since it had the worst rust damage....this is typically done to keep fuel from running out of the gas tank fill hole on the drivers side when tank is full, so im told).
- I got drain holes in the bottom of the cab mounts now also, after having seen the above.

The coating process I used was (and again take it for what its worth, my choice of materials was based on having something local I could get to when I had money and time):
- Clean off all major rust/dirt (and within reason minor rust/dirt) with 3m wheel attached to drill on frame and elsewhere. Then blow off and clean up that. (Acetone/etc).
- Used Rustoleum "Heavy Rust Primer" from Home Depot type places. It comes in a gallon container at a reasonable price and I wasn't afraid to waste it. I put it on with small rollers, brushes, whatever was necessary. I even poured it right down the cowl vents (before I sealed them) and let it run on down out the cowl drain into a bucket. Then let dry.
- 3m in the aerosol can for undercoating over the primer. Easy to get at parts store, tried to buy the more expensive version. Cost to coverage is probably actually horrible as a can really doesnt go that far. The product seemed to do well but like any almost "tar like" product could smear or scrape up a bit under enough pressure (like putting the bed back on the frame) where the rustoleum seemed to really hang on. Over all I liked it but again, no real world test, cost to coverage not so great.
- Used a "rust converter" in any areas it seemed necessary that werent getting coated with heavy rust primer/etc
- sealed any joint/weld everything with good quality silicone, then made sure factory drain holes were open and clean, then through trial/testing opened up any necessary drain holes of my own. Nothing seems to stick to the silicone real well (no surprise) if you are rubbing anything against it (floor board welds, etc) but all areas around silicone are still coated and silicone not rubbed on stays coated. Ill re-coat before finalizing whatever I put down in the interior; sound insulator, whatever.
- Home application bed liner in bed, with my own drain holes where needed.
- I believe some areas of new metal (new floor pans/etc) I didnt always use the rustoleum but just sprayed with undercoating as the rustoleum seemed to not really be necessary/stick as well as it did to rusty metal. Play by ear.

I can see how someone would recommend just spraying/painting. I painted the truck with single stage urethane and whatever type primer you use with that. That seems to have really held up well. (Of course I went through a ton of trouble to give it a good base). Ive blasted it point blank in a few spots with my pressure sprayer that will remove small amounts of concrete from my work truck and the paint showed no issues.

The rustoleum seems to have a good stretchy attribute once its dry when put on thick enough. The 3m claimed "rubberized" or something if I remember right; it had more that tar like attribute, where it might stretch a bit, get everywhere, and if you get some gas on it or something smear. I thought about using a material we used to use to waterproof concrete basements, about eighty dollars for five gallons, it was real rubber, wasnt sure how itd stick to metal.

Some of the other guys on here will probably have some way better recommendations, specialty products, etc. This is just how I wound up doing it. Figure I guess we'll see how it goes.

 

The one I am restoring came from the factory with no undercoating. Only painted completely. No primer showing. It was undercoated at the point of delivery at the railyard in Laurel MT. There was a place there that pre delivered vehicles prior to being loaded on the transport trucks. Father in law had it undercoated at that point. In my opinion as in others. I am keeping mine just painted as I also feel that undercoating has the potential to trap water in areas that can not drain due to coming in from some high area and not being able to get out at the bottom.