I was turned onto WD-40 salt belt rust proofing by my brother who had been using it on his F-250 for years. They are known for bed rot around the wheel wells. I'm in Massachusetts btw, super salty winter roads. I've heard of old timers keeping rigs solid for decades using diesel, kerosene, bar lube, motor oil whatever. I also figured this stuff will wash off, not so good for the roads or environment, well it doesn't wash off to my surprise.
Lets face it, the biggest problem for 3rd gen salt belt owners is rust...and rust proofing is kinda like that thing that seems like a burden chore. There are several different methods of rust proofing, but I like the results and quick ease of WD-40.
Getting down to micro scientific. WD-40 is very thin, it penetrates very small scale and fissures. It will infuse with small and large scale rust. With the static of road driving it attracts dust particles and over time it builds a grime barrier. This grime barrier is very effective in stopping the damage salt does. Any areas I spray with WD-40 turn a blackish color over time, the orange rust turns black. Its not so oily, just like a grime infusion.
If you drive a clean untreated 3rd gen in my region for one Winter, you can see the salt damage taking effect in one year. My 99 is clean and not rusting using this seasonal method. I like the fact you can spray WD-40 on everywhere.
HOW I DO IT - Make sure rig is cold, you don't want hot exhaust pipes. Items I use, compressor, paint sprayer, 1 gallon WD-40 from Lowes, 1 spray can of WD-40, painters plastic laid under rig, face mask/respirator for fumes, rubber gloves. First with spray can and small red hose I spray thru rocker panel drain holes, door drain holes, hatch drain holes, hood drain holes. Next I fire up compressor and spray entire inside frame, then outside of frame. Then I spray the complete undercarriage from front to back, thru open hood also, and even the core support face. Then I let it drip off for a few hours. When you start it up, it will smoke a little til it burns off the exhaust. It will look wet for some days til it does its thing and evaporation takes effect.
MAINTENANCE - Don't go thru car wash, the high pressure jets with soap will break down the coating over time. Just top wash it, if you want to clean the undercarriage, drive fast thru a big puddle. Seasonal rustproofing this way will build a nice protective film on your undercarriage, just never wash your undercarriage again. Fluid Film and other wax based coatings get ripped up by car washes too. I have been doing the WD-40 treatment twice per season, once at 1st snow and then around February, but I could probably do it only once because of my nice acquired film coating.
SEE THESE PICS - Notice my fingers rubbing on the frame coating yesterday this November from a coating that lasted all the way from last February, so you can see its stay put effectiveness. See frame grime. The underbelly floor is supposed to be grey, notice the blackish color. Anything I spray with WD-40 turns black over time. The wet black frame pic is right after I coated it.
OVERALL - WD-40, Its not for everybody, but it does work. Also know that the region you live in and how you drive may dictate your method. I drive a lot of miles, my rig is running everyday, that keeps the frame and undercarriage dry and aired out. If you have your rig parked a lot in a wet region, it will build up with moisture and rust just sitting there.

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Many ATV'ers spray wd40 all underneath their quads before a true Mud ride! Makes cleanup a breeze....

It's useful on anything except plain dirt situations, like say on chains, it will help dirt stick and grind things down fast. Or just move to the southwest! Luck all.

G'donya mate. I think the issue with parking it in a wet environment is if there's any residual salt still underneath. It will absorb the moisture from the air and continue rusting. My moms is parked long term in the weather but I cleaned all the salt off so the rusting dramatically slowed.

Is it better to drive dusty roads after applying wd40?

So are you saying you've been driving that thing in the winter it's whole life like that? Impressive

That rig looks great for Mass. My cousin lives in Pawtucket, gets his undercarriage doused in Fluid Film....

Nice. I live in Upper Michigan and we see plenty of salt. I've used Fluid Film the past few years on my other cars with good results but I might try this too. I bought the spray gun kit from Fluid Film in addition to the 32" wand to stick in the frame rails. It's satisfying once you know the underside has some protection. It seems like the passenger side rusts out faster in the rust belt and I figure the sloppy water, salt and slush mix runs off to shoulder since most roads have a bit of a crown to them so the passenger side gets hit the hardest with salt.

Another lube I tried was Maxima chain wax that I use on my motorcycles. It might be better for the warmer months but I spray the punch welds, rocker panels and when I replaced the door speakers in our Camry I coated the bottom half of the inner door with the wax before putting the panel back on. There were no weird smells and I figured it couldn't hurt. Water seems to bead up pretty well on it.

We don't have a garage to fit our vehicles but we just put up a 20'wx20'L Versa Tube car port a month ago and I was happy to have it the first time it poored rain and now the snow is here.

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That truck looks CLEAN! I use Amsoil metal protector (stuff is really strong once it dries and it stays on there for good) and also Fluid Film AR in the frame, etc. (parts that need some protecting but won't get washed off easily from daily driving).

Comprehensive Corrosion Test: 46 Products Compared | Day At The Range Looks like WD-40 Specialist is performing better than WD-40 for corrosion protection.

I don't wash my undercarriage at all.
No I just drive normal roads after application. It will pick up micro dust from static. I wouldn't want a silt build up. I just sprayed it yesterday, its setting up nice while I'm driving around and its airing out.

Its a full life in MA rig as far as I know, I bought fairly clean with 139K, I'm at 333K now a few years in. Actually looks better now than when I bought it. I know these things good as I've had a few plus a couple Tacomas and know how quickly they can get trashed from salt. As long as the coating is intact it wont rust. Key is as with any kind of rust proof treatment you give it, don't drive thru high pressure car wash, keep coating intact. What I'm doing works great, my brother does it also, as do others.

Currently waiting for my gallon of Fluid Film to show up and it will be my first time using it. Normally, I have used diesel motor oil that I spray my frame with once a year, but I'm trying to be more environmentally friendly this time around.

Does a gallon of WD-40 coat the entire frame with some left over? Or do you have to buy a gallon of it each year?

Yeah I can imagine diesel has a strong smell too, was it doing at good protection though?
WD-40 laid on right, and and given time to set up will not harm the environment, it stays on. My coating is still on from last season. You'll def not use over a gallon, if you lay it on too heavy it will just drip off anyway. I'd say about 2-3 quarts is plenty depending on how heavy you spray, with some drip off on tarp. Plus the spray can for rockers, doors, hatch and hood lip.

Nice post. Thanks for the pics and details.

The used diesel oil works very well. Very thick and black, so it leaves a shiny black finish.

Good to know about the WD-40 though. Cost me around $50/gallon of Fluid Film. Looks like a gallon of WD-40 is around $30. We shall see how the Fluid Film holds up.

Follow up. Here's a frame pic 2 days after coating with evaporation and set up done. Will pick up some micro dust and dry out more and keep the grime barrier staying this way throughout the year. F'ing hate rust, not happening on my rig.

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