I just picked up a 2006 SR5. It is super clean inside and out, but the car came from Mass. and seems to have accelerated undercarriage rust. I imagine due to 3 winters and no washing I really need this car to last a long time or I wouldn't be too concerned. I have searched threads and found some talk about coatings. Anybody have experience with an easy treatment that will stop the rust and prevent further corrosion (DIY or professional)? Results???

I had same thing, 03 4Runner, very clean outside, but very rusty under. I did coated all parts with matte Wurh rust proof and undercarriage paint. Looks much better now. I guess rust is common on 4runners, because when you drive on dirty roads or salt, entire engine bay and bottom of the car is becomes very dirty. Toyota did not put any splash shields there, cheap car anyway.

I just purchased an 04 v8 limited. Mine has the same issue and it's pretty extensive throughout the frame underneath. the service team said that I should swap the frame from a old parted car and put it on mine. I'm curious as to how your situation turned out and if you have any suggestions please let me know thank you!

any luck with this? curious as to how it turned out. I have alot of rust on my 04 Limited v8. thank you!

I was/am in a similar state, although I have addressed it for the most part. Rust isn't something you can just wave a magic wand at and let it be gone. Let me give you some tips, also, post a few pics of the rust.

1) Take a large mallet or large hammer and get under the car and start wacking the frame with it. It feel solid when you hit it, almost like you're hitting a gong where it wants to reverberate. If it sounds soft or you feel like you're going to dent it, then the rust there on the interior is significant. You will want to start making a mental note of how bad it is as you go along. If there are a lot of areas where you look at it and think "this is really bad, this may not be safe to drive" then just swap the frame if you can do it. You can still buy a whole frame from Toyota for ~4k. Yea, that's a lot but it may be worth it. You could consider hiring a welder to weld areas that are really bad but I'd advise just getting a new frame. I myself, am contemplating it but I probably wont since my rust has been addressed and isn't bad. I trust my frame enough to carry around my 2 year old if that says anything.

2) If you determine that most of the rust is surface rust and your frame is solid still, then do this:
start scraping the rust off underneath using a drill (portable probably best, but if you have one that has a nice long cable then that works too). Attach to the drill this kind of wire brush...
Amazon.com: DEWALT Wire Brush, Knotted, 1-Inch (DW4902): Home Improvement
Amazon.com: Katzco Wire Wheels Brush - 2 Pack Knotted and Crimped Cups for Rust Removal, Corrosion and Paint - Hardened Steel Wire for Reduced Wire Breakage and Longer Life: Home Improvement

I linked two, but the key here is that twisted wire adapter. The other adapters are TRASH, dont buy them. Won't work for what you want to do and you'll just ruin them.

You're going to get under there and spend a freaking week taking off as much rust as you can until you see some shiny metal. Another item that is awesome for this, is a needler. Rent it if you can, it will just knock that rust right off and gets in some tight spots. Just gotta be careful with a needler when you're around brake lines and such. This process will take awhile and will be exhausting but it is required if you don't want to just swap the frame.

What this wont do, is get the rust on the interior of the frame... BUT.. a large mallot... wacking the frame all over will knock that rust loose really good. Then you will want an air compressor to blast the rust out the drainage holes.

Don't be lazy, take your time with all of this. I spend about 5 days under my truck for 4-8 hours a day just cleaning the rust out.

Once you get the rust away down to as much shiney metal as your forearms can bare, it's time to start using some products on them.

First, I suggest spraying your whole frame with a degreaser such as GREASED LIGHTNING. Buy a gallon: Amazon.com: Greased Lightning 204HDT All Purpose Cleaner/Degreaser 128 oz (2 pack): Health & Personal Care

You can pour it into any spray bottle. Spray the whole frame inside and out as best you can. Go overboard. SPRAY THAT SHIT GOOD. As you're spraying, get some of those large green brillo pads and wipe it around as best you can. We have to remove as much grease and grime as possible before the next step. This is a very important step, you don't want that crud on your frame when you try using a rust converter.

After you got it all sprayer and have wiped it down, hose it down. Rinse off the greased lightning... THEN DO IT AGAIN! Yes that's right, do it again.

After you got the second round done, it's time to use a rust converter. I suggest using Corroseal. Amazon.com: Corroseal Water-Based Rust Converter Metal Primer, Rust Converter - 1 Gal,82331: Industrial & Scientific
You could also use OSPHO. Amazon.com: THE SKYBRYTE COMPANY 1275 OSPHO GAL Metal Treatment Coating, 1 Gallon, Green: Home Improvement

But I prefer Corroseal. Both work, OSHPO is a little runny so it's hard to keep it on there. IMO, OSPHO would be better if you can dip your frame in it but you can't.

The key to corroseal is to use a SPONGE. Like a sponge you'd use to wash your car with. Get a couple sponges since they get dirty fast. Corroseal will convert surface rust to an innate metal and then coat it. You will have to coat the frame with corroseal 2-3 times but in the end it will be nice and shiney and resistant to water.

That's it, it will take you awhile to do all of this and I would only do this if you're able to leave your 4runner jacked up at least a foot or more for a few days and have all the necessary tools and time. I check my frame bi-yearly after doing the above and it still looks really good. There are areas on my frame where I can see that I didnt remove the grime or rust good enough, and the rust continues to grow underneath the corroseal layer. So I just grind it back down in that spot and repeat the above steps on that spot. It seems to be doing very well now.

The key is really getting that current rust off it. Remember you'll never do a great job on the interior rust of the frame. I do jsut spray the corroseal or OSPHO on the inside of the frame occasionally just to try and slow the rust down. It works.

Amazon.com: FPPO 3PCS 3 Inch Knotted Wire Wheel Cup Brush and Twist End Brush Set,1/4" Shank for Drill,Perfect For Rust Removal, Corrosion and Paint: Home Improvement

This may be a better buy for your drill. The one I use is the 3" knotted steel but I could see them all being useful. I also forgot you can use an angle grinder to remove rust too, but I never tried that.

just an FYI: this is an 11 year old thread & the 2 members you are asking if they had any luck havent been on here for 10+ years. there is a search feature you can search any topic you need, frame rust is a very common thing discussed on here countless times.

Thank you for this write-up. Super helpful.

"FYI." I actually did use that feature why does it matter to you how old the post is? I replied to a relevant thread. The response was valuable and positive. Other members found it helpful. Did you contribute to the actual context of this thread or are you just being critical?

sorry man, was just letting you know the members youre asking for updates on havent been active here for 10+ years. however i have contributed to quite a few current threads on frame rust.

I had never thought much about frame rust; it was just something that was there on all vehicles and as they got old, you just bought a new vehicle and it never mattered. I now have two 4th Gen 4Runners, and I regard them as permanent vehicles, especially the 2007 V8 4WD Sport Edition which isn't made anymore and seems to be somewhat rare, so I am inspired to care for the frames just like the rest of the vehicle. I have bought some POR-15 (never used it before) and I started on the frame of the 2009 yesterday. I got worried because the bumper hitch looked severely rusted, and the front brake dust shield is half rusted away. I realized I had never been underneath the vehicle since I bought it in 2017. I have the back bumper cover off exposing the rear of the frame which turned out to be only slight to moderately rusted; the differential case looks the worst, but solid. The trailer hitch, which is most visible looked bad; I brushed the flaking rust off of it and the rust turned out to be thin and on the surface; the trailer hitch is solid; I am removing it, rust converting it, and putting it back on. I was worried about the trailer hitch having heard of a trailer hitch, safety chains, break-away cable and all, breaking off the tow vehicle and the trailer rolling away. I will work forward and coat all the rust I can get to. After a day of this, I am concerned about the getting lazy part; this isn't really fun work. I plan to make touch up an annual event.

be sure to read the por 15 instructions & use proper prep steps for it.... it is not designed or intended to apply over existing paint or coatings, it is only for rusted metal or properly prepped bare metal.

for coating most/all of the frame with existing factory coating it best to use a good paint like rustoleum or other brand. wire brush or clean the rusted areas & coat with rust converter or the por, then coat over the rest of the frame with a good quality paint. ive used por15 quite a bit on older muscle car restorations & it works good on rusted metal as its designed, por stands for "paint over rust," but on bare metal it doesnt adhere that well & has started to peel & flake off compared to rustoleum or other quality paints.

por is good stuff when used as intended but is very picky on how/where its applied & also is not UV resistant.

I dunno how much salt is used on the roads in your area but here it is used heavily in the winter. All the holes in my frame have been from the inside out.

Interestingly, in 2017, I went to my local Toyota dealer and said I wanted a 2007-2009 4Runner. He told me I was out of luck; nobody with those models was selling them; he could not find anything available. I went on-line and found a one-owner w/100k mi in New Hampshire. I never really checked it out until this week; It seems to have only light to moderate rust. I peered inside the frame with a flashlight, and so far, it appears to be light rust inside the frame.

Here in Cincinnati, they salt the roads pretty heavily, but we don't really have too many heavy snow days. Anyway, I think I kind of lucked out, and I want to maintain what I've got.

Completely understandable. I'm in a somewhat less fortunate situation than you, but my frame appears to be in good shape and a lot has been done to attack it.

Not as much as many on this forum would recommend, but what is realistic for me at this time. I also wish to maintain my v8 4runner essentially forever or until it becomes cheaper to buy another one that's better. Which will likely never happen :P