I wanted to thank you all for the replies again. I am eager to start working on the car Saturday. However it seems I need a bit more research on the final product to buy and the best way/steps to applying them over the affected areas. So my question is:

After I grind all the loose rust off and get a good idea at what I am looking at, will it be ok to leave it bare/exposed like that for about a week? It is likely the time it will take for a product to arrive such as The Right Stuff #3000 ( I am only able to work on the car on saturdays due to my busy work week). Other products would only take two days such as POR15 from Amazon.

I guess I'm still sort of stuck on what is the best product after I get most the rust off and application to keep the rust from coming back.

 

Iron is mined as iron oxide (rust) and refined and mixed with carbon to produce steel. Steel is strong and cheap so use it for car bodies. A beautiful paint job is a great selling point in the showroom but paint's real purpose is to seal oxygen and water away from steel because steel wants to go right back to what it was made from which is rust.

Certain types of rust can purposely be produced on the surface of steel to provide limited protection in a low humidity environment. The bluing on firearms is an example. Rust converter is also an example.

Besides paint, waxes and oils are used to seal steel away from oxygen and water.

 

Sealing out the air is obviously best, however if you are going to have to do things over multiple days as long as there isn't road salt etc involved, I'd leave it bare while you work.

The point of it being that it is MUCH easier to clean off flash rust than it is to remove paint and pick up where you left off.
Also, any phosphorous conversion application effectively results in a non rusting medium/coating anyway.

So if you need to scrape one day, grind the next, chemical treat a third day etc, it's no real harm. Just be sure to do a wire brush clean up before each new attack so you aren't working with road grime and dust etc in there.
For your situation though where you are going to be better off doing some substantial disassembly for better access, i'd just rent a car while doing it so there's no pressure, plus if you find out something worse while there (or a bolt breaks and adds more complexity to the day) you aren't freaking out about your transportation the next day.

Earlier you quoted my post and asked about doing the above followed by por15; I don't use por15 but many people do and swear by it, which is enough for me to consider it valid.
I personally only do a conversion treatment then a wax based rust preventative coating like from ThreeBond unless it is a painted surface (exterior paint surface or otherwise always exposed surface) in which case proper priming, filling, and paint coating is in order.

 

I may have read every rust thread on this forum because, I too, am dealing with said rear frame rust similar to the OP's. First thing you should do is bring it into a Mitsubishi dealer and have this reported. There was a recall issued last year about the front subframe rust in association with front lower control arm bolt failure due to said rust conditions. This EXACT rear frame rust above the rear subframe is becoming more and more common as our cars age. Mitsubishi and the NHTSA should be informed about the condition of these vehicles (like discussed with the rear bumper reinforcement rust).

https://www.evolutionm.net/forums/ev...a-merge-8.html - see rust thread, page 8 has similar post to this thread.

Simply put, this rusting of CT9A unibodies is a design failure by the manufacturer. Absolutely ZERO rust preventative coating used with a sealed frame rail are careless flaws in engineering to either cut cost or weight in this sports car. Mitsubishi is notorious for poor quality control of their vehicle structures. No secret here.

The worst part of this rust issue is the fact that this vehicle is unibody, and the location of said rust. Best way to handle this is dropping the entire rear subframe out of the vehicle to gain total access to cancer spot, which is difficult for those who don't have body or shop type access to do yourself or pay thousands for a restoration shop to handle. This rust is a tricky one for those affected who care about the condition of their EVOs. If enough report or have Mitsu aware, perhaps they'll do something about it (wishful thinking).

 

If you think there's a safety concern you need to fill out a complaint on the NHTSA website. If there are sufficient complaints or a large enough body count, NHTSA will assign an investigator. Your local Mitsubishi dealer is not in this loop except when it comes time to perform a recall.

 

 

this is actually a good alternative because the subframes are usually the sections that come under attack first and they are inexpensive enough that swapping them out for a new one wont break most people's banks. Mine has a couple of spots so I will try to clean them up first. If that doesn't come out to my expectations I will simply replace it. Some can be had for as cheap as $200.

 

Rust is a hard fix. I am seeing rust in areas where suspension arms are connected to the chassis. Sidenote: Has the toe ever been adjusted or are the toe arm bolts seized into place?

If the rust has gone through that much metal then removing the rust will compromise the structural integrity. The rear subframe and gas tank are probably hiding more areas of rust. My vote is to replace the rear floorpan and related members and arms. MN150936

https://www.mitsubishipartscentral.c...r-pan-mn150936

 

 

My car (as well as probably every other Evo driven in the rust belt) exhibits similar characteristics. Ironically, my body is rust-free. Unfortunately I don't think any amount of grinding and painting (with the rear suspension still in the car) is going to do much good ... unless you can coat the subframe inside and out, you are only buying some time.

I had my front subframe replaced as part of the recall ... I wonder if the rear subframe rust has the potential to be a recall as well ... there is potential for the whole rear suspension to come out if that attachment point fails ... that could be a safety issue.

I think Kevin is right in his thinking ... perhaps if enough of us go on the NHTSA website with our VIN #'s along with descriptive photos, maybe they will do an investigation. It can't hurt. Again, they recalled the front crossmember:

https://www-odi.nhtsa.dot.gov/VehicleComplaint/

 

The thing is replacing the front subframe is easy. The rear subframe is AL so thats not going to rust but rather all the suspension attachment points are what is problematic there. Once that starts going there is no simple bolt on solution. Im in the process of cleaning up my front core support and areas around that; cant believe how rusty mine is getting

Once I get the car back up in the air Im going to do what I can to treat the undercarriage. The key is using a product that seals it but also doesnt lock in moisture if you cant grind down to bare metal. I know people have had good luck with POR15 so I might explore that route. Curious what people are using that is more of a permanent fix.

 

Someone on here had a Mitsu part number for the rear subframe rails. Correct, the rear axle / subframe is aluminum, but it bolts to these rusty, potentially dangerous steel rails. Hence why I think it has recall potential.

Nothing wrong with trying to fix it, but it only took about 5 minutes to file a complaint on the NHTSA website. Again it can't hurt.

The link I attached previously didn't work. Here it is again ... I'd encourage all Evo owners to fill it out:

www-odi.nhtsa.dot.gov/VehicleComplaint/

 

Maybe I'm not using the correct terminology ... I know the rear aluminum axle / suspension cradle / subframe bolts to the steel unibody, and the rails in the unibody are what are rusting (and I have seen a Mitsu part number for these rails, probably as a collision part) and would have to be welded / bonded in. Not an easy task by any means, but again, that's what I think is needed to fix this issue properly.

 

Admittedly I didnt read the whole thread. If those rails can be sourced then that is cool albeit as you said they need to be burned in which should a recall come out thats not exactly something a local mitsu dealer is going to be capable of doing and certify.

And even then thats just the frame rails...I wonder what other parts Mitsu offers in this department? Rust scares me; I spent my entire weekend grinding my rad core support to get rid of the rust and coat it before its too late.

 

Last I checked just about every single part of the chassis is still available but they are whole sections of unibody and beyond the scope of most body shops nowadays since the insurance industry has moved to just totalling everything once unibody is involved in accidents... And so few actually repair rust.
A recall is not going to happen realistically. It's an environmental issue on a component (unibody) that is treated as a whole after chassis assembly, not like the k member where there was improper preassembly treatment from the vendor.
If this were happening to vehicles not exposed to corrosive road treatments that would be one thing, but it doesn't. As long as vehicles are made of steel, rust will happen if you put them under duress.
You should see some of the northern Escapes/Tributes... Even the southern ones rust badly, but the northern ones rust out so badly the suspension literally comes straight through the floor, the wheel wells and rear shock towers rot out entirely... Not like Evo rust where you see layers eroding, but entire square-foot sections of metal just completely gone. Ford did a recall to add a brace to keep the shocks attached before even considering actual repair of the unibody. And those were rusting out inside of two years, not 15 years and comparably minimal rust (when viewed as a whole but cherry picking the worst examples) issues.