Oil as undercoating to slow down rust

Jan 3, 2020
In France we seldom use road salt, especially in the west of the country where I live but my W124 comes from belgium and I have a rusty jacking point and rust in one wheel arch to fix which is a classic on older Mercedes. Other than that, the car is solid, I had a good poke around with a screwdriver and I can only find surface rust underneath the car. I'd like to keep it from rusting even more by spraying something oily and I'd like to avoid anything rubbery that can trap moisture if cut/hit by rocks.

Here are the (cheap) options I found so far:

- WD40 : seems way too thin, I think it would evaporate in hours.
- New motor oil : still too thin to cling for a long term, bad for motorbikes if it drips all over the road.
- Used motor oil : same problems, even worse for my health and the environment but free.
- Chainsaw oil : seems like the best option, would definitely cling. A bit too thick to seep in every crevices. What about mixing it with something like mineral spirit or kerosene to spray it? If the lighter components evaporate while leaving the thick oil everywhere, this would be a nice and cheap solution. Seems to work as well as Fluid Film or Krown for many people.

I don't want to spent too much money on other products as I like to tinker and try and I'm cheap. I don't mind if it's a tacky mess under my car, I hate rust.
Rust never sleeps. Fix the rusted areas and remove the surface rust first, paint or otherwise cover those areas, then spray with oil. Some people here say used motor oil works as well as anything else.

In the Rust Belt areas of the US certain shops are set up for spraying oil on undercarriages. This is normally done annually in the fall before winter weather begins.
Do a search on here for undercoating, a lot of previous posts.

Another option is Cosmoline / I like RP342
It's a waxy liquid that will NOT wash off.

"I'd like to avoid anything rubbery that can trap moisture if cut/hit by rocks".
Cosmoline is NOT like this.

Fluid Film - enclosed areas / doors, hood, tailgate, trunk
Cosmoline - high wash areas

Edit: One thing to be aware of is, an oil may have an effect on 'rubber' components, causing them to soften.
Also, any undercoating oil you use, make sure it will not affect electrical connections.
It's best to use a name brand product, and read the instructions.
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I like to work smarter, not harder. Oil as others said will wash off and require frequent re-applications. IMO that's not working smart. Use a product designed for rustproofing that isn't going to wash off easily and doesn't require as many re-applications. That's working smarter, and doing a better job.
IMO oil is going to drip and make a mess. See if you can buy Fluidfilm online. I used it this year on my Ram and I did not see any drips on my garage floor. It seemed to work very well. I got the idea from others on this site.

just my $0.02
I've begun going to a "hot oil undercoating" shop in Shrewsbury, VT.
They say their mix is oil and a thickener. Locals and regular customers say the service is very effective.
Might it be possible to paint on oil and then "cook" it on with a heat gun or something? I'm thinking about what happens to cast iron pans if that could be something on underbody parts. I know it is a wild arse thought.
I had a coworker who was cheap and wasted nothing. He drove a 1967 Ford Galaxie that was amazingly rust free for living it's entire life in the suburbs of Chicago. In the Winter he would save his old motor oil in one of those old metal, pump up bug sprayers, and spray it everywhere under his car.

The fender wells, frame, and all of the undercarriage were soaked with the stuff. If it got wet and sloppy out after a Winter storm he would spray more of it. (He kept the can full of it inside his basement, next to the furnace so it was nice and warm, and would spray easily).

After he did it his exhaust and muffler would smoke like crazy until it burned off. He never had a fire. Lucky I guess.
The areas you point out are positioned such that just normal driving will blast most coatings off over time. You'll either need something very tacky like some of the Noxudal products, or prep and coat those areas with a paint product.
Thanks for your opinion. I'm going to look for a product made for this purpose. Krown, Fluid Film and Texaco Rust Proof Compound L keep appearing on bitog. I'll try to find something similar here. Maybe there are great products in marine shops?
I wonder how spraying liquid paraffin, (melted wax), would work? You would need a way to keep it liquid while spraying it. Then it would set up and coat the exposed metal, preventing rust.
I like and use Waxoyl, although none of my toys see road salt but the Rovers do see the beach. It is big with the old Rover community, is waxy, self-healing to an extent, and doesn't seem to 'trap' rust like rubber undercoating and doesn't abrade off too quickly. Downsides are cost and mess; messy to apply, sticky and messy to work on. Works very well for its purpose though.

In an ideal world you'd first convert all of the rust to a stable phosphate compound. Then cover the formerly rusted area with primer and paint or some other permanent coating. A POR-15 kit (+/- paint) accomplishes all of the above. Finally spray with Fluid Film for good measure.
All on your list will work, but will wash off too quickly. You also need to be aware of the odor that whatever your using will give off. I tried 90W Gear Oil once on an old truck I had, but the smell was terrible.
I finally found some crude oil (PA Bright) that didn't smell at all, and it worked great. I used it on all the door/hood/trunk metal seams and it slowly seeps into the hidden areas.
My long dead uncle was a jokester (or maybe a Smart Alec - take your pick). He used to say that when you saw a freshly oiled road instead of slowing down to keep the oil/tar off your paintwork you should drive as fast as possible to get a "free undercoating".

But I imagine the effort to get all that oil/tar off your paintwork would have more than made up for any benefit!

I never saw any evidence that he had actually done this himself - he was just setting up any gullible listeners.
While many here are referring to slowing rust on the exposed-to-wash-off exterior areas, the worst problem in my opinion are the weld seams and boxed in areas. In the salt belt most of the sheet metal rust starts on the inside and works its way to the outside, initially appearing as bubbles under the paint .