I have noticed some rust under the boot lid on my NC, it’s in a small area around the area with the lights and catch.
Images:
My questions are:
Thanks in advance
Just rub it down mate, i believe the bootlid is aluminum.
Sand to remove as much or little of corrosion as desired.
Prime and paint
It depends really on how decent you want it to look as an end result, but presuming you’re avoiding welding work or a replacement boot lid, then the DIY process would be along the lines of:
Decide whether you want to do the job with the boot lid on or off the car. If off the car I’d recommend a blanket under it so as to avoid scratching it as you move it round. If in-situ you may wish to newspaper up the areas of the boot area and lid you won’t be working on to avoid rust dust, paint splashes etc.
Remove the lamps and blanking plate.
Use an electric drill on a power you can control with a wire brush attachment to sand back the visible rusty metal. Be aware that the metal round the lamp/blanking plate photo looks thin, don’t be surprised if it goes to large holes (this may have already happened removing the lamps and blanking plate).
Use emery cloth of different graded grits (rough to smooth) to smooth out the sanded metal.
Also double up some coarse grit paper and rub it up the inside of the air gap (presuming there is one) around the area where you removed the lamps/blanking plate from as deep as you’re able to go.
Now use Rust Remover on a sponge that you can squeeze up this air gap, as well as on the exposed surface round where the removed fittings were.
Once this is done, use Anti-Rust paint and again get it into every nook and cranny you can, and again coat the exposed outer metal with at least one coat.
Once the paint has dried, if required, use filler to build up any missing metal work to rebuild where the lamps and blanking plates go.
Skim a very small amount filler over the exposed metal work, keep sanding it back with paper after each application until it’s smoothed flush with the surrounding metal work, you will likely cause some scuffing to the surounding paint.
Apply more coats of Anti-Rust paint over all the exposed metal and filler.
Once the Anti-Rust paint as dried, apply coats your Mazdas paint colour over the Anti-Rust paint.
Once dry you may wish to spray rust inhibitor/cavity wax/etc. before refitting the lamps/blanking plate.
Looking at the rust, my guess is water is getting in round those fittings and once trapped is rusting out the lid, if this is the case some sealant round them wouldn’t go amiss.
Every so often remove the lamps/blanking plate and spray in some rust inhibitor/cavity wax to delay for as long as possible the rust happening again.
brilliant write up, thank you, will give me something to do over a weekend.
PHRT boot lid is steel, aluminium is the softop one. You really need to rub down to bare metal, clean prime and respray. And it will go off again because it will have rust from the inside.
Eventually it’s a new lid job and figure out how to get a good seal on the lights
It’s a steel skin on the boot lids of PRHT’s aluminium skin on the soft tops.
So you’ll need a rust converter to paint on first to treat that. Not so bad on the ali boot lids although it needs grinding back to bare metal where possible.
The underside there isn’t really treated with a proper lacquer coat so prone to paint peel the n corrosion.
Did mine a few years ago on the red NC…
All done and a few coats of lacquer added to finish it off properly. This was on an aluminium boot lid.
Something like https://www.halfords.com/motoring/paints-and-body-repair/rust-removal-and-treatment/nitromors-rust-remover-non-hazardous-500ml-768062.html would be ideal then?
There are loads on the market and many to suggest, all much claim the same type of treatment.
For smaller jobs, no need to spend loads unless you’re doing rusty bits elsewhere on the NC (we won’t go there)…
would the 90ml be enough for my needs? didn’t want to end up with not enough.
Personally I’d go for a rust remover (e.g. Jenolite 150g Rust Remover Thick Liquid | Jenolite but any that works would do) these days over a converter.
In either case, you’re sanding away most of the rust so you’re not dealing with much which is why the smallest amount is fine for a job like this. The same goes for the anti-rust paint (I prefer Rustins because a little goes a long, long way).
Also, now that @MickAP posted a photo of the lid with the lights etc. out, presuming the steel one is identical it may be easier to have some thin metal pre-painted to bond to the inside of the boot lid to strengthen the areas you’ve thinned/removed through sanding the rust away so the eventual filler just sits on top. I originally thought the internal air gap would be fairly small based on your photo, but his photo makes it appear quite large.
The one downside is that then you have two pieces of metal pressed together, which is a recipe for rust if moisture gets between them, but sadly as @ckleanth pointed out, you’re only delaying the rust and eventually you’ll either need real metal work done or a new lid (but that could be years away).
It goes a long way, you’ll only use a fraction of that Kurust in the small container on such a job.
I’ve got a selection, the larger container has 1/3rd left and I’ve treated the undersides of the last NC.
BTW get an old credit card or something like (plastic) and slide it under the left side of the light lense push it gently then it should lever out. Same on the centre blanking plate.
Thankfully I haven’t got that problem on the white NC (steel lid) it has had cavity wax sprayed in there previously.
When repaired, make sure you clean and dry it each time you clean the car. normally missed and then dirt builds up, collects water and aids corrosion.
Done the same job on mine this week . But mine was a aluminium boot lid. went over it with wet and dry . Two coats of primer filler light rub down then two coats grey primer. Light rub down four coats of top coat then two light coats of clear lacquer. Take care when you remove the number plate lights one of mine broke . When you remove the lights you need to push them to the car’s N/S with a trim remover tool under the lip of light . Don’t use red primers as these will blead through the top coats.
These threads really gets to me as everyone is concentrating on " whats visual " only , and if it as fresh paint and shiny then its all good , resolved…
Its far from it on the mx5 boot lids of any age and mark ( waiting to see if the MK 4’s have the same issue yet).
The Eunos, Roadster,MIaia,MX-5 all suffer from condensation in the boot areas as a trait of the cars design from the MK 1 on wards , which means it gathers on the inner skin of the lid and runs down to where the skins are folded together where there is no paint for any kind of protection against bare metal where it sits slowly attacking in the seam hidden away until it swells and pops( just like the rear arches) , and there is nothing you can do to put it right against the shiny paint then , its to late, it is time to bin it and search for a better one.
So pop the light units out etc , get the kurust in there to treat, (because they will be rust in there and oxidizing if ally) and then kill all the inner edges with a cavity wax, ones in a can will do and get the correct protection in there, top up every 2/3 years, that will save you boot lids and shiny paint.
M-m
Thanks MM.
Any chance of going into even more detail of exactly where the rust will be forming and how to get the Kurust and cavity wax in there.
A full on how to would-be soooo useful
It’s very easy on the boot lid, you can poke cables through the framework for such things as reverse cams so an extension from a can of Dinatrol cavity wax will be a doddle. There are many holes too to poke in at, notice how Mazda just tape over those holes with a small painted dot.
I will bud ,I’ll do a guide.
M-m
Yes, common problem on MXs with the steel boot lid.
I have a 2009 NC2 which I remedied a couple of years ago. It’s obviously a poor design by Mazda, but that doesn’t help you.
Remove the lights and plastic blank plate and then it’s down to how you would normally deal with a rusty area. Once complete buy some Dinitrol ML (it comes with a long applicator tube) and spray liberally within the concealed areas of the boot lid, spraying into the welded seals. The ML will leak out immediately so cover the boot well and have some rags to hand. It will also continue to leak out in small amounts over the months that follow but is easily wiped off with a dry rag.
I took the view that this area isn’t really in view so the finish doesn’t really need to be immaculate (that’s my opinion others may disagree). It’s all about getting rid of the rust. You’ll still need to keep an eye on the area in the future.
Good luck,
Dave
Fantastic 
