I spent 20 years on ships made of steel. In saltwater and high temperature heat, once it gets a foothold you're in trouble. There are chemical conversions you can do but they all start with mechanically removing as much rust as you can. Even things like POR 15 won't totally stop it. Best bet is to prevent it as much as possible to begin with.
Heat is a catalyst for rust. I have a heated garage but my Tacoma has spent its 10 years of life sitting in the driveway during the winter. Because that 5 months of the year that the roads are salted here it is usually below freezing. Heck much of it below zero °F for that matter. Not a lot of rust going on at those temps. One reason the Taco frames rot so bad around the Cat is the heat that they produce. Even in the cold.
When new and first couple of years every fall I sprayed the frame with Corrosion Free Rust Cure Formula 3000. It is what the Canadian military decided on after testing a lot of CRC products. After a couple of years I went to Fluid Film because it was easier and cheaper to get. There are a lot of people in the North that have FF parties this time of year to spray their frames.
Must have worked for me, I got the recall notice on the Tacoma for the Toyota CRC treatment this summer. When I picked it up the Tech came out and asked what I did to the frame, he said it was in the best condition he had seen for the age. He said they did spray it though.
Picking up a 2019 GX460 tomorrow. Carfax showed it had been serviced in Mn. during the summer and Texas in the Winter so it hasn't seen salt or at least much of it. One of the main reasons I considered getting it plus likely an older snowbird couple owned it on the 4 year lease. I'm getting too old to climb around under a vehicle spraying the frame so next week will be taking it to a shop near me that does oil undercoating. They use NHOU Oil Undercoating and I will have them do it every Fall. CRC 3000 (a.k.a. Canadian Snake Oil LOL), Fluid Film, NH OU Oil, doesn't matter so much. Just Get R Done in the Fall. From the first year and every year going forward. Stay away from hard or rubber under coatings. Far, far away.
I'll consider Corrosion Free Rust Cure Formula 3000, thanks. I forgot to mention Dinitrol AV30 is highly toxic, so careful adherence to the MSDS would be appropriate.
Fluid Film is safe for pretty much any application. My Weber Genesis grill at only 6 years rotted out at many points. Weber was good at replacing the rusted parts, enough that I got replacement parts for free from them.
As usual if you want something done right do it yourself IMO.
So, what would you recommend for the underbody of my 2007 Landcruiser ?
Fluid film, then removing the rust (how?) then paint and fluid film again ?
Good advice here - I'd also encourage the Fluid Film idea though I personally use Woolwax which is the same kind of Lanolin-based coating, just a thicker product that will hang a bit better than FF (good luck finding cans of Woolwax though - may have to go w/ compressed air set-up as they're having difficulty with their vendor for aerosols). I'm in MN so we're deep in the rust belt and I've tried POR-15 and Chassis Saver with mediocre results - that's mostly due to air/moisture will almost always find a way through the coating due to missed spots, scratches, or areas where the parent metal/chassis wasn't cleaned enough.
The thing w/ Fluid Film/Woolwax is that you just need to clean and re-apply at least annually. Well, I suppose you don't "need" to clean but you're probably going to have the best results spraying on a clean surface and you can get a good look at what's going on with the chassis on an annual basis. By "cleaning", you could pressure wash or just run a sprinkler under your truck in a few areas for about 10-15 mins each. With that said, you'll just have to get used to not using car washes during the winter, so you don't blast all the film off of your truck. You just have to trust the film. If you want to find a provider that does this, there may be shops that specifically mention Fluid Film or Woolwax treatment but there's also Krown which is more of a Canadian company, but I recall some have been popping up in the US.
I'll note that if you already have some kind of soft rubberized undercoating (3M aerosol or factory), I think the Lanolin stuff may soften/deteriorate that material. Not really a big deal as I'll spray over any of this factory undercoating that has been scratched anyway as I don't want the bare metal exposed.
Oh, and I suggest not doing the hard rubberized undercoating as that stuff can crack/split or be punctured and end up acting like a pocket for moisture/salt to get underneath. Also rubberized undercoating is awful to deal with when trying to disassemble components that have been sprayed with this stuff. For funsies, here's a vid f/ Paul w/ South Main Auto (he's a mechanic in New York state so salt/corrosion in a recurring theme in his repair work).
Great discussion. I remember Ziebart undercoating from growing up in north east Ohio. We need to find another chloride chemistry to use on the roads in the mid west and north eastern USA. I have to report keeping my Jeep Wrangler TJ free of major corrosion for 23 years required no effort in western Washington.