That’s tough as you had done the underseal and cavity waxing. Good thing is you now know, can get the repair sorted and have a good look/retreatment inside the sill(s) (is it both sides?) whilst the work is being done.
Is the rest of the body underside/steel components still good after the treatment in 2018? I hope so.
Good luck, worth doing properly as you are keeping the car and will be happy when you are still banging around A road roundabouts and B road corners!!
Your supposed to apply wax coatings etc every few years so thats why most fail really, they are not a miracle product sadly.
Some products are better than others too so it depends what you used and what you painted over.
Is this the fate of all MX5’s, including NDs?
What if you don’t drive in the wet (or salt) but the car lives on the drive, but always has a half cover on so nothing does down those drains? Is that then immune, or still just delaying?
It was 9yrs old when you had it treated so past it’s best by then anyway. As far as Mazda was concerned it was time to scrap it and buy a new one.
Any treatment at that time is going to be applied on top of rust so whilst it slows it down, it’s already bitten by then.
If you want a rust free car then don’t buy a Mazda, but that’s not why we do it!
Some NCs are on their second set of cill repairs, so take that as a warning and get it done properly.
My S-Limited Mk1, which was treated with dintrol when imported by a fastidious owner who then hardly drove it, lasted 14 years before the sills and arches were shot. I can attest that the dinitrol was everywhere.
My S-Special, which was driven everywhere, and put on 200k kms, was never treated with anything, lasted 17 years from import before the arches and sills were shot.
I am a great fan of Dinitrol ML. I have used this product in all my NCs. My first one was WU56 WSL in 2019. She was sold to a chap in Edinburgh, and so far, she has survived six Edinburgh winters. I remove all the wheel linings and then the grommets. With the extension hose, I empty around five cans into the inner sills, subframes and chassis rails. Any possible rusting areas on the floor are ground back to bare metal and treated. My recent addition is to remove the trim over over the lower seat belt anchorage and treat the box section.
Sorry to hear that rust has taken a hold on your NC. It really is a great shame that rust is such a big issue for all mx-5s. The speed at which it becomes an issue will clearly vary; age, usage, previous rust prevention treatment all relevant. Rust never sleeps unfortunately, but it shouldn’t stop us enjoying our cars. I hope you get it sorted and have many more years of enjoyment.
Oldest ones will be coming up to 9+ years old so would be interesting to get a good look at one that has had an un pampered life to see how they’re holding up.
Allegedly some of the metals used in the manufacture were upgrades from previous generations.
My 6yo one which has done half it’s life in the South of England and half in NE Scotland is about due some preventive treatment this year, so far it’s looking pretty clean though the subframes would benefit from treatment now.
I seem to remember that the ‘upgrade’ to the corrosion resistance was part way through the life of the ND, so the state of early NDs may not be reflected in later models. Perhaps one of the more knowledgeable could recall the reference to this change.
Amazes me how accepting MX5 owners seem to be about the scandalous lack of rustproofing on their cars, up to NCs anyway. Mine is a garage pampered sunny days car which only gets wet when washed but there’s still visible surface rust underneath (viewed at MoT time).
Cheap affordable sports car was designed for global market - rust is a significant problem in about 3 locations in the world - and not a huge priority for the car’s production meaning the protection on them is geared for the wide audience, rather than a more specific one.
Sadly though one of the affected regions is the 3rd largest market for the car - however, that is why the cars are dirt cheap in the aftermarket and you just have to accept that fixing the rust in the UK is an ongoing maintenance issue!
Slapping underseal over the car when it is box fresh is about the only step that can help you fend off the issue longer, but it’s going to get there eventually with the atmosphere. Means only the most cared for examples in humidity controlled spaces are likely to go on for the long, long term.
Sadly if the process has already begun, no amount of underseal and injection is going to stop it.
The ND hits 10 years this year, NAs and NBs started showing issues at around 11 years NCs about the same, so I’d expect to start hearing about ND problems if they are going to emerge - in the next couple of years.
