What to do about this rust? NA8

Morning all,

See below some pictures of rust on both rear rocker panel skins. What are your opinions on how bad this is and what I should do about it? This is far far from the worst I have seen and probably even further for a lot of you guys. Still, I want to catch it early. The car came over from Japan last summer and was absolutely spotless when I got it in the autumn. I had it up on a lift two weeks ago and still looks spotless underneath. It is just the two mentioned area that are showing some rust. Now I am hoping its just the outer skin, and due to the condition of the rest of the car I am think that’s likely?

What would you do? Does it need a new section of skin? Or can I get away with a wire brush, treatment and some paint?

I am planning to get everything under sealed this summer!

Hiya, as you said, there’s a lot worse out there! If the arches are fine, no bubbling etc, personally, I would locally treat these areas with rust converter after removing any loose stuff, fill and repaint the black bits. A tidy, temporary repair could be done for very little money. Unfortunately, when they go, they go and you won’t cure it by the method I suggested, only slow it down a bit and postpone the inevitable. You would at least get this year out of it, maybe longer and give yourself chance to sort a decent repair shop out and set aside the funds to pay for it. Sounds like it’s well worth doing though, fresh import and all that. 


If there is rust on the outside then the question is what is it like on the inside? Money no object I would take it to  https://www.themx5restorer.co.uk/services/welding-repairs/rear-sill-repairs/     and let them have a look. they have a nice channel on Youtube https://www.youtube.com/watch?v=9Ef0BeYsHAo&t=436s

Thanks Barrie, this was my first thought. Never done it before mind!

Yeah I will check the inside when I do some preventative work. I took my car to Skuzzle recently and they said it’s really clean underneath. Hopefully that’s a testament to the rocker panels. I reckon its just salty roads corroding the outside (all fingers and toes are crossed.) As Bettabuilda said, it will be worth doing a good temporary repair whilst I save for more substantial work… I understand they all need doing at some point!



I am thinking of using Hammerite stuff. Rust Converter and black paint. Any objections or better recommendations?

Arches? At first sight relatively superficial and easily dealt with as you say.


The beginning of the end for them by the time it erupts outside. They rot from the inside out. Quickly if unchecked.

To me, they look like a previous repair…so how well was that done.

In the final analysis, I’m afraid you won’t know till you at least get a borescope in there, but I can g’tee the only way to stabilise sills is to cut out the oxide cancer, insert new metal, and inject anti rot fluids. Just make sure the drain-holes are included. 

I’ve seen worse.


Yours looks like a flesh wound in comparision.



As a “temporary” thing, the kurust should be fine, it s cheap, you can get it at your local store too, get a Matt or satin black, easy to put on and matches more like OEM finish. Unfortunately they never rust from the outside! It’s always the other way round so what you see has found a way out from the inside! As scottishfiver has said really. He just types quicker than me 


Thanks Scottishfiver. It looks like some money saving is going to be needed.  

Cheers Barrie. She’s being a bit of a money pit at the moment!


Don’t fret too much.

I had a couple of cheap temporary welds done to satisfy MOTs until I knew I’d retire with it and get stuck in properly.

You can poodle along quite nicely doing “Elastoplast” mini-fixes for a bit.

We’ve all done that from time to time…in my case starting with 60’s Minis as a callow yoof.

Bubbling on sills; there is more to come through. Treating from the outside is a waste of time and effort. As it has bubbled, pinholes have already formed. If you research, rusting sills in Japan is already a frequent occurence. The rust hasn’t started because the car has been in the UK.


Japanese owner moaning about the endemic of NA and NB rusting. Welcome to the party:


I sincerely believe the days of getting a guaranteed rust free NA from Japan are over. Time has caught up on these cars. That’s not to say there aren’t rust free cars still there, but there is no way to guarantee it.

I would leave it alone, and try and forget about it for a year or so. Regular photos will allow you to keep an eye on things. Keep the wheel arches clear and clean, and you’ll have a good chance of saving them (debateable whether rusting starts in the non-draining pocket, then tracks back up to the arch, or the arch seal fails, allowing dirty road water to pool in the non-draining pocket).

This Japanese owner gets it


Right now, there would be a temptation to do a minimal repair, when likely, you will be back at it. There are various treatments you could try, but its difficult to get these into the non-draining area where this rust has occurred. At best these will slow the rusting. They won’t fix the hole that is already there, and the danger is, as you frantically try and pump wax in there, you will create sink spots that will encourage pooling of condensate, and ironically accelerate rusting. Better off focussing on clean water chassis washing, and sealing the wing lip. Storing the car in a garage with the boot open I think helps as well.

Thanks guys, I feel frustrated but weirdly comforted by your help 

I think i’ll treat the arches and steer clear of the sills for now. I’ll try and keep the sills clean and clear of rubbish until I can justify getting them done properly.

This diagram is good - 


Their rust inspection sounds good, I think I’m being a bit incautious because there’s nothing obvious. Bit of a drive to get there but I’m sure they’d point out anything else that’s obvious too, and presumably their rust prevention treatment is good as well.

Yes, that diagram from Saz’s last link IS good - and illustrates pretty simply why it’s VERY difficult to get effective rustproofing where it’s needed on the NA and NB.

That narrowing gap between the lower front of the rear wing skin, and the inner sill, is where our sill-killing rust starts.  Its easy enough to see, in cross-section, how moisture (water that shouldn’t be in there, or even just condensation in a ‘never used in the wet’ car) will creep by osmosis right down into where those panels join (and where our marque-defining corrosion start).  It’s equally easy to see the difficulty of getting treatment down to very bottom of that narrowing cavity - in practice, with the complex structure in this area, access to even start is almost impossible.  In through behind the seat belt trims, with an extension probe backwards and navigated through the structure -  on a VERY hot summer’s day, is about as good as you’ll get. 

My '96 Eunos was an import from Japan in 2015.  It appears mint at the moment - kept in a garage and ‘never used in the wet’.  But - I’m under no illusions… 


…oh, and I second Saz’s opinion about keeping the bootlid open - in a garage obvs - if you’re lucky enough to have access to one. When my car is laid up over the winter, I also strategically place very large bags of silica gel around the car.  Two get pushed up as far as they will go, either side inside the boot, above and in front of the rear wheel arches.  Not sure how efficacious they are - but guess they can’t do any harm. 


There are other places just as good, but they don’t have the catchy website address. Their repairs are no better than others who have earned a good reputation, and I think they cut their warranty to one that is inline with any repair (its not a warranty against rust coming back, but against MOT failure).


My black S-Limited had been extensively dinitrolled by a previous owner (still dripping with the stuff inside the doors). It did not prevent the sills needing welding. Any rust inspection is going to tell you you have rust (you know that already). From the inside, you will have a patch of rust metal about 10x the size of your rust bubbled. The rust bubble starts as a pin hole in the metal, water escapes from the cavity and starts to lift the mastic-paint. Early on, you can actually peel back the rubbery paint, and be greated with incredibly clean metal, and think you have no rust. But look closely, and at the centre is a tiny black spot, sometimes with a blob of water on it.  Thats the pinhole.



Old picture from 2009, my first rust hole. Small blob of paint. I pushed on it, and my thumb went through. Minor repair needed. That is my 1996 S-Special, imported in 2000 (by Mazda), no additional rustproofing, small repair in 2009, by 2017 (and with nearly 270k kms on the clock), both sills and arches cut out and replaced. Car was mostly never garaged. Arguably suddenly got worse when it started to be gararaged regularly after 2014, but who knows (also, the repair was a bit of a bodge, but lasted until 2017, so happy with 8 years, which is as long as MX5restorer claims for his (thereabouts)).


S-Limited: Imported in 2002 (1993 car), dinitrol in cavities upon import, used as a barely driven show car until 2009, daily commuter 2009-2013, barely driven since then (just sold it). In 2018, needed two sills and one arch.


I’m going to conclude from that, that Dinitrol was largely ineffective in slowing down the rust. Probably gave the car an extra 2-3 years before rust got a hold.

Rust inspection service is alright if you think you have no rust. But otherwise, its a waste of money. Ditto trying to rustproof rust. Companies will be more than happy to take your money off you.


Are you saying I effectively cannot get my car rust-proofed in a way that will work? Not even a place like mx5restorer who have substantial practice and will take pieces apart to do it?

The car was largely undriven for many years and shows no signs, as far as I know it does not have rust. But I feel like I’m hijacking this discussion a bit, I’ll leave it for another.


Indeed, and also illustrates that “blocked drains” are the root cause  as being a red herring (I’ve seen some owners try and blame other owners for rust, because apparently they didn’t clean their car enough).


Relatively few repairs in the floor area, and usually only on completely rotted cars, where the inner sill has gone. If it had been blocked drains all along, then the floor would go first.


Its just a feature of the design; a complex inner structure, with inevitable rust traps. I wouldn’t even call it design error (same for Mk2). Its just usual. Salt on our roads has accelerated it. As a nation, we probably don’t use chassis washes as much as we should; much more common place in Scotland and Northern Ireland (Northern Ireland though is driven more by the fact the inspector will fail your car if the underside is dirty. And, at one time anyhow, there was a 4 week wait for MOTs).