Just reading through the miata.net article. Brilliant stuff!
I was wondering though, cannot the seams towards the wheel arch and the bottom of the sills be seam welded to make them water tight?
Obviously I have no clue about the limits of welding, but it seems like spot welding will Always leave gaps which is doomed to invite water in. The reason it’s used by the car industry is because it’s fast, not because it is water tight after all 
Also, that dark gunk which seems to be used for sealing along the wheel arch, why can it not also be used around the rest of the sills/arches (if you can find something suitable to use)?
Or does it interfere with fitting and welding? 
You can buy seam sealers, various brands, but make sure the seam you are sealing is clear of rust first.
The way it seems to me, this issue comes down to the following:
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Moisture/dirt/salt etc… comes In to the cavity between the outer plates. How do we prevent this or minimise this?
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The mess from above cannot get Out properly from the cavity. How do we improve this ability?
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For the mess that Is already in the cavity, how do we minimise the impact it has i.e. prevent it from causing rust?
Regarding number 1. how it gets in there: As I see it this is the Main Problem as all the other problems would be less if this didn’t happen. Entry seems to be Mainly through the seams between the plates, probably through capillary force and/or from being Pushed in there by the speed of the spray thrown at the seams, for example by the rotating rear wheels. Partially it is also moisture/condensation from constant temperature change which cannot be prevented. Is it also coming in through any of the existing drain holes? It does not seem so since the rust seems to mainly start in the outside section which does not have any drain holes (part of the problem probably). So to mimise the incoming mess you could for example (opinions please):
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Apply sealant along the whole outside seam when new sills/arches are installed. Apparently sealant is already in place from factory around the rest of the wheel arch. What is the possibility/suitability for doing this?
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Actually Seal the seams by welding when new sills/arches are installed. What is the possibility/suitability for doing this?
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Alternatively cover the seams with some cover (plastic, metal?) but it seems like the consencus is that nature would defeat this as any mess would creep in below any cover anyway? But maybe some “spray cover” similar to the plastic wheel arch liners could be fashioned to cover at least the seams exposed to the rear wheels, a bit like mud flaps, but inside the wheel arches? It would leave the seams still openly exposed to the weather but get them away from Direct spray from the wheels, which othewise seems to be the case?
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Apply Rust Prevention gunk (Waxoil, Dinitrol etc…) directly on top of the exosed seams. The question is: Will it stick? If is exposed to the rear wheel spray it probably wears off cquickly. It may also be unsightly, particulary on the side sills, or is it situated low enough to be a non-issue? What is the possibility/suitability for doing this?
Regarding number 2. how to get it out of there: There currently seems to be No escape route for the gunk, it gets in but has no way way out. The outer cavity does not seem to have any Drain holes and since it is a Very tight space and is located mainly in a visible section on the lower side of the panels/sills it may not be practical to add any drains there?
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Could drains be added Inwards of the cavity, to help drain into the already existing drains for the rest of the sills?
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Could the cavity be made “wider” (for example inwards) to make it easier to Reach through existing “inspection” holes? That way Perhaps the cavity could be reached for cleaning by brush, spray or whatever. The problem with this is still that there would be nowhere for the gunk to go, unless you can perhaps “lift” it out of the cavity, but that will surely be very hard.
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When the inside panel is repaired, can some inwards holes be added, not so much for draining as for “airing”? The cavity seems so tight that the gunk at the bottom of it is really squeezed and never gets a chance to “dry”. Some added holes to the inside panels would perhaps at least allow air to reach this section which should make the gunk less “keen” on rusting?
Regarding number 3. How to minimise the impact of the gunk: For the gunk which Does get in and does Not get out, how to minimise the impact. This will presumably have to be by applying rust prevention to the Inside of the cavity. Apparently this cavity is Really hard to reach but miatanet suggests having an extra hole made in the wheel arch to enable reaching the cavity with a suitable tool and to make it possible to apply rust prevention Into the cavity.
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Presumably there would be no particular harm in filling up this cavity with Rust prevention gunk? Maybe that will even prevent any outside stuff from entering, since the cavity is already full? However, there will nevertheless probably be some exchange with the outside which will dilute the rust prevention over time so it will need to be re-applied. Could the cavity be filled up with Rust prevention at the same time as the Sills are replaced/repaired, or does it interfere with the welding job? With an added inspection hole in the wheel arch it could still be added separately though.
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Could any holes for reaching the cavity be created from the inside, top, door frame? These holes might be less exposed to the nasty environment in the rear wheel houses.
So, with all these points in mind, is this something that could be made into instructions to whoever repairs my sills next time (Finishingline, Mx5 Restorer…), or is it just ramblings from someone who has no idea what he’s talking about i.e. me and is even exaggerating the problem? 
there is some very interesting discusions going on here, some of you will know of my work with MX5’s and the fact that the core of my work is on repairing rust, mainly in the sill area, so here are my thoughts based on what I have learn’t
the rust is caused by a conbination of poor design, and poor quality materials (metal)
- poor design - it has already been mentioned that the outer skin of the back of the sill is actually a wing, just like your front wing, only in this case it is welded on rather than bolted, it meets the structural sill towards the bottom and follows it to the sill lip, this happens at the vertical seam that runs down the sill from the bottom of the door, if you feel it with your finger you will be able to follow it to the sill lip, and then all the way to the back of the car, the rear wing is spot welded to the structural sill all the way along this seam, the problem is caused by the surface of the wing and structural sill touching each other, this traps moisture and dirt, which traps more moisture, which stays wet all the time, so rust starts, which traps more and more and more.
its worth noting at this point that the hood drains have nothing to do with this, water will always get into the sill void, it is impossible to stop it, the sill drains will also not stop it, because they drain the inner sill section, not this area, obviously making sure both of these drains are clear will help prevent more serious rust, but it will do little to prevent this problem.
- poor quality materials - it seems mazda have cut some corners a the cars have got newer, early mk1’s seem to be the best quality, for example I recently welded both rear sills on an H-reg mk1 UK car, and it was better than most later imports I’ve done, I am doing more work on Mk2 and Mk2.5 cars than mk1’s at the moment, for example I currently have 2 Mk2.5’s in the workshop at the moment, both having the rear of the sills repaired
Now, onto preventative measures, I’ve seen a few comments about drilling drainage holes, well I can’t really see that working, because the problem isn’t really caused by free flowing water that can drain away, its the dirt that gets trapped which gets and stays wet that is causing it, everytime I cut these cars open this dirt is always still wet, so a hole could allow more dirt in, and even more water to keep it wet, also where would you pt these holes, in the area just above the touching faces? I’m not sure it will help, I would fear it could make it worse.
My course of action (and what I do to every car I repair) is to carry out the repair by removing the whole outer lower section, I then remove the surface rust from the inner sill, treat, and protect it, I then weld on a new outer skin, once that is on I then drill a series of holes above the weld line into which I instert a multi directional lance and I spray a good quality cavity wax into the holes, I do this till it starts to run out of the sill between the two skins, this I believe is the best reasonable solution, it won’t cure it, but it will give it the best possible chance, I believe 
I hope the above helps, please feel free to ask me any questions if you have them, and feel free to check out my website
Thanks for the reply Gareth, very interesting!
Having read some more on miatanet it seems clear that drains are probably not an option because there really is not enough space for them. The wing and structure lies on top of each other so it gets Very tight, no real space for drains. Probably also a reason why gunk gets trapped. The plates lying on top of each other means it’s very tight, but neverthless a gap which can hold the gunk tightly so it cannot get out. Probably the closenes of the plates makes it even easier for the rust to get hold of the metal.
Another item (from miatanet), what about adding some holes to the Inner panel structure while it’s available during repairs? Then plug them with rubber plugs. That could help with airing and access from inside the car for future inspections and updates of the rust protection?
Can the external seams between the plates, like at the rear of the sill and in the wheel arch, be sealed? For example using Seam Sealers?
It would be interesting to see if something like this could be used to perhaps help seal the gaps where the gunk enters, perhaps between the sill, wing and structure plate edges. It seems like some of these can even be welded on(!).
Finally, once repaired I plan to have an underbody rust treatment, like this.
Surely that cannot hurt? 
The seams are actually sealed from the factory, and I also seal them with a panel seam sealer when I carry out my repairs, there are soooooo many holes and gaps in the sill though, water and dirt can get in from many area’s.
Answer; fiberglass sillcovers. Forget about rust. What you can’t see, can’t hurt you…
Well, it certainly sounds like your repair methodology should have a better chance then most to prevent the rust from re-appearing.
Considering that you’ve been doing this for a while, do you get many re-visits (from the same car) ?
Do you think adding a couple of more holes to the inside structure, probably in the lower section, would be at all helpful?
Have you got any experience with using other materials, like the fibreglass mentioned?
Presumably the inside structures can still rust? But maybe that is less likely if they haven’t got the outside Metal plates to lean against…
It was a facetious comment. A fair few cars with fiberglass bodykits are probably hiding a world of hurt. On the other hand, rust here can probably be repaired by the most ham-fisted welder, and no one will ever know. Its up there with some of the Japanese and American approaches to repair, which have included chucking fiberglass in the holes, or using filler to bond in a piece of metal.
I’ve not had a single come back from the same car when I have carried out this work.
I do not believe adding extra holes will help
I would consider the use of fibreglass a bodge.
ok guys i have somthing that may be a help to you all
try making sure your rear hood drain pipes are clear
trust me they get blocked so easy
if there blocked the water sits in the inside of your sills and rots them
same for the front drain pipes
if you are usure where they are PM me and i will take some pics for you
Water will still get into the sills even with the pipes free of any blockage, and there are not any roof drainage pipes at the front, the drainage at front is via the rubber that goes around the top of the windscreen and down the front of the door, water should run inside that, and drain out of the bottom into the door shut area.
Yep, most mk1 MR2s are just rust underneath the side skirts, and you can’t pop them off to check if you are trying to buy one. They look better without the skirts - it’s the first thing I did to mine when I bought them, and also lets you get started on fixing the rust.
None of my Toyotas got to 19 years old without lots of welding - my MX5 is holding up very well in comparision.
Hello,
The mx5 i was checking out today had some rust at the bottom of the wheel arch at one side. How bad is this? Can I avoid spending big money on it? It’s a 2004 mk2.5
http://img259.imageshack.us/img259/7648/jobbhatso3.jpg
http://img713.imageshack.us/img713/6994/jobbhatso2.jpg
Assume it needs welding.
can you ellaborate on that a bit? does it need an urgent and expensive fix?
Is this the only part of the sills that are a bit rusty? If so that rust can easily be cut out and a shaped piece rewelded back in. Mine was like that and I did exactly that, I cut it back to be clear of rust and to where the metal was as thick as it should be. It took me about 4 hours to cut out both sides and shape some metal and bend it to the required shape. Fortunately my son is a welder and he spent his lunch hour at work welding it up for me. It then took me another 4 hours to grind the welds back and get it blended in with the rest of the sill. I painted it with rattle cans as my sill is black. 
What is the “rust perforation” warranty for MX5s?
Even my Ford Focus has a 12 year perforation warranty.