I live in NH and rust will rot out the frame real quick if untreated. While used car shopping, I looked at countless cars that were so rusty underneath, I would walk away. When we got our 2009 4runner, the first thing I did was to spray fluid film inside and out on the frame and everywhere else underneath. I got 12 cans of fluid film and an attachment with a small hose with a 360 degree nozzle at the end to get inside the frame. PB Blaster has recently come out with a similar product called surface shield. Home Depot has it and it is slightly cheaper than fluid film. It claims to last 2 years so I use it on the exterior of the frame. I'm very impressed with both products and use them religiously because I HATE rust.

Same problem as with the Tacomas?

I picked up the Woolwax brand gun and 5 gallon bucket of fluid film. Did all three of my vehicles this year using up about 2.5 gallons total for all three, one being a large E350 van and the other two 3rd gen 4Runners. It's been about 2 months since application and so far happy with the results.

New Jersey reply here:
My son has a 2002 Tacoma double cab that still had a decent amount of Noxudol 300 on it from the Toyota voluntary recall. We simply washed and reapplied over the entire frame. Not wanting to apply a different product over the top of it. We used Noxudol 700 cavity wax inside of the frame rails as well.

When my 04 tacoma got a new frame, I chose to treat it using the cavity wax (above) on the inside of the rails and this product RP342: Cosmoline Vehicle Undercoatings - Long Lasting Rust Protection for Vehicles everywhere else on the frame and underside of the body.

So far each is holding up ok, occasional touch-ups as needed. As was suggested in comment above. A self healing product is best.

Yeah, New York 4runner owner here too. The roads today looked like a forest fire was up ahead there was so much salt dust flying around. Not good for any car….

Anyone have any experience with Waxoyl? Sounds pretty good

I am sold on Fluid Film for undercoating. I hate the sealers that basically just lock moisture around the steel of the frame and cause more problems than doing nothing. I think i used about half a gallon on my runner but thats completly spraying the interior of the boxed frame and splooging the entire undercarriage.

DIY writeup here Ultimate oil undercoating guide (DIY Waxoyl)

Downside as mentioned is that it softens on the areas that get warm, mostly on shock valves and brake fittings in my case, which basically reduces it to the efficacy of your typical room temp applied fluid film/etc. In a bad new England winter those need to be applied twice a season. In most areas this waxoyl lasts for about 5 years. My frame and everything above is still covered but a few areas are getting crusty and could use a touch up so in the spring I'll be hosing it down and respraying. I may try cosmoline on the fittings and driveshaft.

Anything that goes on at room temperature and doesn't contain solvents taht evaporate out will wash off at room temperature in 1/2 - 2 seasons depending on your area.

Cosmoline is a good product that doesn't get enough recognition.

IMO, an oil-based, creeping/self-healing is the best by far.

Fluid film, krown, wd-40, or even used ATF are all superb. In a sprayer and properly applied, you can keep any truck pretty much rust free with not too much time invested by treating your undercarriage with one of these. Personally I like fluid film because I think it's a bit better for the environment than ATF or wd-40. Krown is probably similar but haven't used it.

That being said, any treatment is more environmentally friendly than scrapping your car due to rust and the idea is that the majority of it sticks to your frame anyway.

After not so good experiences with several other products (Ziebart, cough, cough) and lots of research, I went with Waxoyl. I like what I read and what I see now that it's applied. Does it work? Give me a few years to see.

I was considering wool wax, from what I've read it's the same as fluid film but doesn't wash off as easy. The wool wax dealer says full application 2 years in a row then touch ups for the rest of the years.
The Waxoyl dealer said one application then annual reapplications in wear spots. Waxoyl adds some sound deadening as well.

Not sure which is better.

Once temps start warming up here I will be using Corroseal as the rust converter followed by a protective layer of Rustoleum then protected with RP-342 Cosmoline.