Rust on sills and on components behind rear side panels - more cost effective options

  1. My model of MX-5 is: mk1
  2. I’m based near: Cambridge
  3. I’m looking for technical help or recommendations on: Advice on direction for expensive repairs

Hi all,

My car is currently with a good UK MX5 restoration specialist getting the area around and behind the rear sills de-rusted, however on finding the extent of the damage the price has risen beyond what I can afford, approx 3k. The garage has been good and honest, they just work to a high standard, and a larger area around / behind the rear panels require work than expected. They have also suggested the possibility of repairing only the significantly rust damaged areas to lower the cost, but have been clear that the remaining rust would spread to any new plating within around 1.5 years.

I am happy to prioritise removing and preventing rust over how the car will look at this point. I am considering having the car transported back to me so I can spend a bit more time looking at alternative options. Does anyone think it would be likely that I could get a less polished repair which would stop the issue well but cost less?

I’m struggling to find any diagrams of the car around this area too, would anyone have any links to resources where I could find out what the parts behind the body panels at the rear sides connect to, and in what way they are structural?

Thanks in advance,

Try an internet search for ‘mx5 restorer video’ and take your pick.


£3k sounds excessive for typical rear sill/wheel arch repairs.

A lot of the cost will be in paint and finishing.

If the garage has already cut back metal to reveal damage, you are already in the hole for their time getting to that point. Which might make it easier if you recover the car and get a mobile welder to finish.

You’ll like need

Outer sill/arch repair panel (2x per side. If the arch needs cutting out, the inner arch might have gone, and some shops will sacrifice a repair panel to form a new inner arch.

Sill repairs; the actual sill might be damaged, it can be patched with scrap. The jacking point lip is not part of the outer panel. Its the structural part; if the sill is compromised, the lip won’t bare weight, and will turn in.
Sill end plate
Floor repairs might be needed

A welder might tack on the bits you need, and then leave all the filling to you.

Repairs are cost effective; they add value to the car.

Without seeing it we don’t know how bad it is. If it’s just in the arches I agree it should be less than 3k but the rust is increasingly going further

The OP says rear panels but do they mean that or when it was cut open was it very messy?

This the well known car throttle pic - a late model 1.8 is like mine. We know can get like this - mine was a bit worse as the rust was higher up the strengthener and the repair was a lot more than 3k.

And we’ve seen pictures Scottish Fiver posted up of the back of his car - what was left of it before what must have been epic work

Thank you for the info, this is really clear and useful! I’ve ordered the car to be transported back to mine to buy myself more time, where I can have a closer took at where the rust is, and if damage is mostly on structural components or not. As you said, it may be there’s someone local who can do the specialist stuff, and I can take on the easier parts.

Thank you for the advice, it’s useful to hear some examples of how the cost can scale up depending on the location and severity of the rust. I’m getting the car delivered back to mine so I can better understand its state, and I’ll make a plan from what I see then

Have you had your car delivered back yet. It’s useful for all of us to get updates on experience of where rust has got to and cost even though it can be painful for the author as I know from my own.

Hi there, the car will be dropped off to me today, so I’ll be able to get some photos uploaded soon. I’ve also been recommended some local welders, so I’ll add the sort of quotes I get back as well for context

If you’re planning to keep the car for a long time and don’t rely on it as your main car I’d consider saving up and get it all done in one shot.

I think a lot of people have a car repaired on a relatively tight budget which often involves rear arches, outer sill replacement / repair, maybe patches to the floor to sill, inner reinforcement - and for a year or so the car looks fine. Then the floor-sill section rots through (usually from the back and jacking pressings) and the inner sill reinforcement is found to be totally gone… or something like that… At that point the only way to properly repair it is remove all the previous repairs to gain access, the cost becomes prohibitive and car gets scrapped.

Unfortunately the structure of an MX5 sill necessitates being rebuilt from the inside outwards (floor-sill, inner reinforcement, outer sill, sill end plate, rear wing…) because you can’t get access to the inner pieces any way other than from the outside

The MX5 restorer YouTube channel does document a very high standard of workmanship and approach if you’re after videos showing what “good” looks like to use as a reference


I’ll send some pictures with context below, it looks like the rust is worse on the right hand side, but on the left hand side it progresses inwards onto the inner reinforcement a bit. I’ll add quotes I get as well for context against the images in future.

MX5 Mk1


Right Hand Side

From what I can see, the rust on the right hand side is worse than the left. Rust is on the inner arch panel, floor sill and inner sill, with greater damage on the inner and floor sills.

Rusted area right

Right Inner and floor sill

Right inner sill with hand

Right inner arch lower angle

Right inner arch upper angle

Left Hand Side

The left hand side seems to have less rust across the arch and sills, however I can see some rust on a surface behind the inner panel on the reinforcement (see last image ‘Left rust behind inner sill’). I can’t see any rust in this location on the right hand side.

Left rusted area

Left inner sill with hand

Left inner and lower sill

Left inner arch upper angle

Left inner arch lower angle

Rust behind inner sill Left

I can only see last two pics as my iPhone won’t see the embedded jpgs.

The crying shame in this is the outers don’t look too bad but as lumphammer says you can’t get at the inners which do look like they need attention without taking off the outer. And god knows I tried to find a way.

Someone who can see the pics will comment shortly. If there is a way of adjusting the setting at your end pls do.

The fact these are sold out at present possibly tells a story that you aren’t alone - although if you never spoken to mx 5 restorer do - I find them incredibly helpful and kind even when you are buying bits even if you live too far away for them to do the work as I did.

Most of those links you posted are dead links, and not visible. eg.

Thanks for letting me know, I’ve reuploaded the images so they’ll hopefully all be visible now! If not I’ll attach them all as functional links

I can see the images now - I couldn’t earlier.

Looks typical sill arch rust how far up into the arch does it go? Guessing some cutting out of the outer arch too and probably mostly resprayed the rear half of the car.
Done properly I guess not far off that price quoted, a full respray alone, no welding/repair work can cost that.

Looks like its had sill repairs previously too.

I’ve seen worse. Mine was worse (see earlier photos), and it wasn’t £3k with paint to sort it. In fact, my sub-£2k (Thrussington Garage. East Leics, so a MX5 specialist) bill included painting a new rear bumper, sorting out a little hole on a door (where it had rusted under panel sealant), and full two quarters repaint. Ok, a few years ago now (2017) but inflation until recently wasn’t that much.

Thanks - that all looks familiar.

There are possibly plenty MX5s out there with the same level of rust down the floor inner which are being driven around. I have been known to stick my head under MX5s and you do come across them with tabs welded over the inner sill floor section. There are videos of people repairing them like this on you tube.

I didn’t patch. I had the outers cut out completely (worse than yours) the strengthener remade and the inner remade/ repaired and the new outer and then inner arches cut and replaced where necessary and the rear quarter panel patch repair with the repair section from mx5 restorer.

I also had the mx5 restorer beefed up chassis rails put in. The rear quarter, sills and door surround - basically everything up to the screen resprayed in classic red by a professional painter the classic car outfit uses (Ive just had the rest of it done to match). As it was coming back in winter and would have to stand outside I had it all Schutz sprayed. I’ve cursed doing this as I have had to take everything off clean it up again and paint it properly - that said it protected it through that winter.

Breaking down the cost the whole lot was about £8500 inc vat. Take off the chassis rails cut out and welding £1000 the spraying £900 and the schutz £400 and it was £6200 inc vat for all the welding. That was with a classic restorer in Durham that had a couple of TR4As having shell up rebuild ( seems to be one of the things they are famous for) a stag, a Lamborghini Urraco, an MG Magnette (the pre war one ) and an MG RV8 in I can recall so they clearly are liked by someone.

Remember for those who think this repair goes to the b post it may not - I had my full outer sills off and work done behind it to the inners which had rust to varying degrees their entire length as did the strengthener. Where you can see rust in the floor of inner on that last pic - mine was like that right down the length of the car. Because you can’t get at from the inside you have have the entire outer off to solve a problem on the inner so if it’s working it’s way down the inner you have to either accept you’ll be doing it again in a few years or do all of it now. Both sides.

Honestly that is not that bad but it does need doing. It looks like the arches have been done at some point and it’s got a few patches but the outer sill is original because of the light grey e-coat. The floor to sill might be alright, what’s the sill look like from underneath the car?

In my opinion that looks like a car worth spending some money on, it’s a low mileage Mk1. You spend it once… if you get a good job it could last 20 years.

Andrew, the other option is that they’re sold out because it’s a PITA job to do and the panels are the wrong thickness especially for use as a jacking point. But they do at least fit unlike others.

That’s the question - how far forward is it running.

I can now Jack off my new girder like chassis rails.

The sill jacking points should only be a last resort if you’re stuck at the roadside with the OEM scissor jack but yes the diff and the chassis rail reinforcement pieces are the best to use for normal servicing

Edit: The point of my comment is that we can’t chose when we get a puncture many miles from home and a 1mm thick repair section with a flat profile is highly likely to deform with the pressure of the OEM scissor jack