Hi all,
Been looking for a clean 3rd gen for a while and have been having a harder time finding one than I expected. Most everything up here in WI seems to have a mostly clean body, with visible rust on the hitch and rear bumper. This seems to translate to a pretty rusty underbody (frame/suspension linkage/etc), but the extent varies. Photos of clean sheet metal can be deceiving. My question is, how can you know where to draw the line regarding frame rust? Any spots to focus on beyond the rear control arm mounts?
Thanks!

The thin chrome bumpers rot out pretty easy, can have a clean frame and rear bumper can be junk. Looking underneath you can tell very quickly if it is not a good purchase. Judge the overall patina, but certainly the rear LCA is the hot spot. A little spotty surface rust isn't an issue, but the majority of frame should be black. Frames rot from inside, and heavy rust can be tell tale sign for thin spots in frame.
In the salt belt would have to have a rig that had good conditions or care. I'm in the MA salt belt and looking near coast line I have better luck. I find that areas that see less snow and less road treatments have the better condition 3rd gens, even then its still a battle, but I do find them. I seasonally rust proof my rig, I don't have a rust issues. I'm guessing WI is tough area to find good ones. If no luck, venture south in your hunt.

Expand your search to the western US. I've got x2 3rd gen 4runners that are rust free.

Fellow WI rusty 4runner owner here. My truck has it's fair share of rust. When I first got my truck a few years ago, I took the time to knock off as much rust as possible and I put on a heavy coat of Rustoleum rust reformer. Needless to say, most of that has since peeled off and she's rusty again. I simply do not have the time nor will to do that all over again so I've just been coating everything with LPS 3 twice a year. She ain't pretty, but she's holding together fine....for now. If I can get a few more years out of her, I'll be happy with it.

My front bumper also has a few holes in it from rust. It will be hard to find a clean rust free (or very little rust) 4R pretty much anywhere in the midwest. If the owner from day one meticulously cleaned and rust prevented the undercarriage, it will look great but they are insanely rare to find. You are probably better off looking down south or out west.

My truck had rust but wasn't rotting, big difference. The rockers are beginning to "rot" but that is pretty much the only area that is "bad". The frame near the control arms is usually the weakest spot that rots through first. If you find one that looks to be ok rust wise on face value, you'll want to poke around with a hammer/screwdriver. These frames often rot from the inside out so poking around is your best bet. If you do find one that has some rust but isn't rotting, spend a weekend cleaning everything up real good and use something like POR 15 to put a new coating on the frame. Then, twice a year, apply whatever rust preventative you like (Fluid film, LPS 3, WD40 etc etc).

Given your area some rust will be normal. When I went to purchase my 4runner I spent a good 30-40 mins banging on the frame with a mallet in different spots. All seemed well so I decided to pick it up.

I've found that POR15 is the only rust preventative that actually works well long term. Rustoleum and those other rust converter products don't do anything and the rust will creep back in again.

Chassis Saver by Monstaliner works well, but surface has to be really ruffed up and/or also it likes to be painted over rusty surface. If you paint over a smooth or glossy surface it will peel.
Overall in the salt belt, if you don't do seasonal rust proofing coatings, like Fluid Film, petro, etc, rust will be doing its thing. I do a full spray of undercarriage, inside frame, inside rockers, inside door bottoms, inside hatch bottoms, inside hood lip. Here's a pic of my 20 yr old frame original paint in salty Massachusetts using only WD40 seasonally. Its all black and solid.

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Wow that's amazing for a frame that's been in a salty area! I was pretty surprised at how small of a can POR15 comes in and how thin it was. But it goes along way and bonded excellent to a surface that was just power brushed.

Mines the same way. I spent a bunch of time trying to get it all gone. The frame is solid but the bumpers have seen better days (also from Moab... took a few hits and slid on some bumpers, but was definitely worthwhile!)

But I agree that rust and rot are very different. Rust is controllable, rot is replaceable.

I like Eastwood rust encapsulator. I bought a jar of it and it goes a long way. Only been a year though and I dont drive in the salt...

I am insanely shocked by this picture, I literally CAN NOT believe my eyes!!! Am I to believe, that you have ONLY used WD40? ever? Are you the original owner? How many miles on your rig? Garage kept? Daily driver?

Sorry I am literally shocked...

I bought it at 139K, there's about 355K on the chassis now. I drive about 1,000 miles per week. Never garaged, driven daily, no matter the salty roads or not, I never wash the undercarriage.
I do my WD40 treatments twice seasonally, once 1st snow, then another for the home stretch about February. It does really work, and yes I only use WD40, see my write up -
SEASONAL RUST PROOFING using WD-40 only

You could buy my 2000 4R limited 2wd from Cali with 200k? Its amazing, frame has no rust nor the body, but is 2wd. Getting 22mpg and is fun to drive an excellent, getting ready to list it.
I also have a 2002 lmt 4wd that is in good shape,
It has a bit of rust, especially in rear by spare tire. It was not gone like some others were, even with 220k. Surface, flaky, can be cleaned up...so thats where I drew the line.
Passed on several that were just too neglected overall by appearance to indicate that maintenance was probably lacking as well.

Now I have 4 Toyotas and one has to go

flabbergasted!!! thanks for the reply and link!!!