How badly do the MK3 rust?
I have seen some videos from the MX5 Restorer on yt and they are seeing more and more MK3 come in with rust issues.
What’s your experience as a MK3 owner or in the trade when it comes to rust on the 2006-2013.
Are the 3.5 (2009 onward) any better?
My first NC was an 06. I part exed it against a 14 NC3 in the summer because it had reached the point of being “beyond economic repair”.
I wasn’t emotionally attached to it so that was fine but it needed at minimum new arches all round, both sills, subframes and subframe mounts, one top mount at the front. Oh, and the boot skin too.
It was going to cost far more to repair and finish than it was worth.
The new one went to Thrussington to be undersealed/void waxed the day I picked it up!
Older mk3 now getting on for 15 years old so like any similar aged well used car, rust will be present. From my experience, worst place is front of the rear wheel arch where the back of the sill meets the wheel arch. The inner arch is covered by a matting that protects against stone impacts but holds any moisture. This is where the problems start. If you inspect one, get your hand in there. Really need wheels off and remove the arch protector but you won’t do this when viewing a car. Here are some sorry pics 
A 3.75 will be newer so I guess less advanced but a high miler might still be prone here
Other areas too but I’ll let others comment
Ouch that looks painful. Seems like a huge oversight to put a moisture holding material there.
Would the hardtop version be less prone to water in the rear sills?
The PHRT have a different system of drainage to soft tops.
The metal surrounding the left hole is left bare, it wasn’t great around the other hole either with just a thin coating of underseal. That is the rust spot, just behind the above mentioned arch liners at the bottom of the rear wing/sill area. Mine has around three coats of Bilt Hamber epoxy mastic applied after grinding back any rust found, fortunately only surface stuff, could have been worse if left. Do the same under the sill too at the rear end especially.
It’s all fixable, I quit enjoyed putting mine right. Lots of protection on the outside and lots of waxoyl inside the sills and any other cavity. But if you don’t want this work keep checking them.
To put it in perspective, that’s nothing compared to a Jaguar S-Type. I would have brought an S-Type R if not for those pesky plastic sill covers. Instead, I settled on a 2004 XJ8, which will need new tank straps soon as one of them is looking a bit twig-like.
I need to get that cover off and look at my boot area. Rust is inside and I am sure that it is coming through from the underside of the boot. Looks like it is riveted on tho
My 2007 NC was waxoyled from (nearly) new and was consequently rust free when I bought it at 57k. Bought a hard top from a guy whose 2006 NC wasn’t, it had done 7k less but it was rotten as a pear…
Yes riveted on, drill the rivets out. My boot well under the car had slight rust bubbling up, grounded back and painted. In that area it only had the thinnest coat of paint from the factory.
So from what I can tell the Mk3 and the Mk2 are pretty similar for rust which is a little bit scary but the main benefit of the MK3 is their age so potentially the rust is much more preventable with a little bit of effort.
I guess many of the MK2 just got so old and the value of them meant that little bit of effort didn’t seem worth it for many.
My 2008 nc is pretty much rust free. But I have crawled under others that were a bit grim.
So worth looking at individual cars , can’t generalise on age or model.
We brought our 07 plate with only light surface rust underneath when it was 6 years or so old. one of the 1st things was to book it in for a full rust protection treatment - still no rust on it and well worth the money on a solid car.
I have a November 2005, and mine is rust free.
Just needed a bit of underseal top up in a few places when I first got it…
I also waxoyl mine whenever I feel the need to tinker.
Mine is an 07 NC1 and needed a little TLC underneath prior to its underseal 4-5yrs ago. A little welding that is.
I have kept on top of any surface blemishes ever since. I would say very individual in terms of rust rate. The older NC1s are approaching 15yrs old now, and likely a couple of owners (though understandably not unusual that folks have them as long termers! 
) so big difference in rot rate if kept outside, driven during the winter months, uncovered, living by seaside, etc versus garaged and used solely as a fine weather car.
Totally agree with Shepp and others approach being to get it rust inspected and protected ASAP as priority #1 after purchasing.
I did find some rust right on the lip of the arches, so will be sorting that out maybe myself.
Tiny spots of rust 1 or 2 and by lip of the archer in mean the bit closest to the suspension. Need to find someone locally who can unseal it and sort the rust maybe. I want to fit full MeisterR coilovers but no point if it’s going to be a rust bucket.
