It's salt season again in Massachusetts. Just did my season opener treatment. Time proven. My underbelly and frame look great, southern style.
Here's what I do. At 1st salting. Get 1 gallon of WD-40, a spray can of WD-40, get out compressor and paint sprayer. Get face mask for fumes, and get to it.
1st, I take a spray can of WD-40, hit all the drain channels thru the holes with the spray nozzle hose. Hood, doors, hatch and rockers. Spray thoroughly til comes out neighboring holes.
Then I fire up the compressor and fill the paint sprayer can with WD-40. I spray in every hole I see in frame, getting all the interior frame. I hit the cross members. I spray the outside of the frame, and other underbelly stuff that can use it.
Of course you need to do it in an area you don't care about overspray.
It really works great it keeps slight trace film year round. It will get In scale crevases if you have them. In salt season I've been doing it one a month, for about 3 times. It's overkill I know, can probably just do it once. But I'm not wanting my 99 4runner to have any rust. I have been surprised how well it works.
I thought WD-40 would be to thin and wash off too easy, it doesn't. Well I don't really wash my truck anyway, but the rain doesn't wash it off.
My brother turned me on to this when he told me he was doing it on his F-250 so it didn't get the all to common side bed wheel well rot.
I know there's other ways, but just wanted to share what I'm doing. It basically works awesome if you're diligent about it. And it's quick and cheap.


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Isn't WD40 petroleum based?

Sure. Some I've heard use bar oil inside frame.

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Good job, there are mineral based oils to use too. There's a company new hampshire...something or other...

Edit: NH Oil Undercoating - https://nhoilundercoating.com/

Cool, thanks. If there's a better way, I'll certainly consider it.

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Is that bad for something ?

Depends on who (or what) we're talking about. It's not going to hurt your undercarriage per se, but it definitely hurts the environment spreading it around.

Underside pics or it didn't happen!

Used Liquid Wrench Lubricating oil (blue can) on lower control arms for 2005 Matrix. It says it's for keeping rust off tools, displaces water, so I figure it'll work on this car because there is some rust but I'll probably not ever put a lot into repairs.

I like your idea, but for the inner frame I am considering spending a little more and might try the Eastwood product or similar. My 2005 4Runner came virtually rust free so it's parked for the winter.

For winter, the Matrix is fine with getting rid of the stock wide low profile tires and going to narrower 15" winter tires, not quite like here's my lifted corolla - Toyota Nation Forum : Toyota Car and Truck Forums, but it does well.

YES! Dinosaurs were harmed in the production! lol

Great advice OP! I've heard of engine oil being painted on the undercarriage by old timers.

I applaud your dedication to keeping it rust free!

I'm a huge proponent of Fluid Film. It's environmentally friendly and is very effective as being a barrier between salt and metal.

I tried WD40 years ago but it always washed off. Fluid film now.

I understand.

That's why I use fluid film. It's more sticky

I used to have a brother-in-law who claimed to be a body man. He swore by using drain oil. Just fill an oil can and start squirting in your doors until it comes out the drain holes. Then park in the street in front of your least favorite neighbor. Not that I would ever do it.

Bad for the environment, as others stated. If I had a house with well water I would not want someone spraying gallons and gallons of WD-40 nearby.

It's also attacks some plastics -- Polycarbonate, ABS, I think polyethylene. I'd avoid spraying it in the engine bay or directly on any plastic parts (including plastic trim pieces). It's not a huge deal or something to freak out over, but avoid it if you can -- especially your headlights!