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I bought my first MX-5 (1992 Eunos V-Special) recently as my first project car. I intend to use it as means to teach myself how to work on cars, and I intend to fix it up and restore it as much as possible.
I've done quiet a bit of research into the rust issues and sill structure of the MK1 and what I understand so far is this (I hope it's correct):
There is an inner sill, an outer sill and the front wings and rear arches overlay the edges of the outer sill. There are 4 drainage holes embedded in the pinch welds under each of the outer sills, 2 near each corner (8 in total). There are also 2 soft top drainage holes behind the seat belt towers, these holes run into a tube which runs down through the inner sill and underneath the car where there is a one way rubber flap stopping debris coming back up.
From what I understand what happens is these drainage holes become blocked (mine were before I cleaned them) and water gathers in the sills, especially in the area between the outer sill and the outer wing/arch panel. Rust starts developing in this area which then spreads beyond that area into the inner sill, up the wheel arch, etc.
The inside of the rear outer sills can be seen via the inspection hole beside the seat belt towers under the plastic trim, and the rear inner sills can be viewed via a small hole covered with a rubber grommet under the wheel arches.
The other day during dry weather I finally got the chance to remove most of the interior and soft top, giving me good access to clean the soft top drainage holes (which were manky!) and gain access to the inspection hole beside the seatbelt towers.
I poked my camera in the passenger side and here's what I found:
And I've made an image with an overview of what I THINK I'm looking at on the passenger side:
The driver side doesn't look as bad as that, but it does have rust in the same areas.
It looks to me that the inner sills have rotted and fallen apart just underneath where the two holes are, and it appears there is rust inside the inner sill which I'm hoping is mostly surface rust, and it looks as if its spreading up the rear wheel arch also.
I haven't got any pictures of the outside at the moment, but it appears there has been a botch repair job here before as the bottom of the wheel arch covering the rear of the outer sill is mostly made up of filler. There is also a very small patch of bubbling on the passenger side arch near the swage line. I've searched for area's that may have rotten through but most of the metal seems solid from the outside.
Also the 2 drainage holes that are meant to be in the pinch welds on the rear passenger side are not there, it looks as I've they've been sealed by the previous dodgy repair.
I live in Ireland, and I'm not aware any MX-5 specialists here that I could take the car to, and I'm not confident that a regular garage would be familiar with them enough to do a proper job. So my plan is to get a MIG welder, practice with it, and then try to tackle this repair myself.
The car is currently stored on my driveway, under a Halfords All Season cover. Its not my daily driver or anything so it won't be driven during bad weather and I take off the cover on dry days to make sure the car is aired out. Unfortunately it will be at least another few months before I have the time, money or the workspace available to attempt this.
So my main goal right now is to minimize its spread between now and when I try the repair.
Would it be a good idea to spray a rust converter like Dinitrol RC900 into the sills now or would I be wasting my time? Also should I drill 2 new holes on the rear passenger side sill seeing as the original holes are gone? And would it be a good idea to put some sort of dehumidifier next to the sill inspection holes inside the car to try keep the moisture level in there down? Any advice ye have would be greatly appreciated!
