As you are applying the process to a new car, it shouldn’t take you too long as there’s obviously no corrosion to deal with first. I did my new 30 AE last March during the first lockdown, soon after purchase, before any build up of road dirt/salt. If there is any sprayed up crud, that’s easy to wash/wipe off first. I haven’t a lift, just trolley jack, axle stands and one pair of ramps (plus a compressor which was only about £100 new, and is useful for loads of other jobs). Many of the areas can be done before you even need to raise the car such as boot access to get to the area between mudguard and rear wing seam, boot lid, bonnet lid (even though aluminium), doors, sills from top edge under scuff plate, and a fair bit behind the front and rear bumpers (remove completely for a thorough job). You can then do all the wheel arches, inside the front chassis rails and the sill/rear wheel arch void using just jack and axle stands. Wheels back on, you can then drive or reverse onto car ramps to access all the orifices in the chassis rails subframe and areas missed by the factory undersealing process. The plastic oversills can be removed either with the car on the ground or with it raised, if you want to do a thorough job behind them.
Doing it this way would allow you to do a bit at a time if you can’t afford the car to be off the road for several consecutive days. If you’re a quick worker with a garage, you may get it done over a weekend. Masking areas such as shock absorbers, brake components and exhaust should not be forgotten.
All very satisfying when YOU know what’s been done.
Thats a very thorough reply - thanks…
A minor point - but related, nonetheless. I observe that directly above the back box on my MX there is what must be a heat shield (held in with 4 bolts?). Again, a little bit of surface rusting on near-side end. Any of you bother taking this off and treating/painting with any product? If yes, what could I use? Thank you.
When it comes to working under a low sports car the MK makes very little difference, to do a proper job the same principles apply of removing covers and accessing voids.
People need to do their own risk assessment.
It is handy to remove that shield for treating both it and the hidden area of chassis rails and boot floor that removal exposes. As it is exposed to radiant heat you would be better off with removing the rust and using a higher temperature paint as underseal is not really ideal for the purpose.
Heatshield is held on with 4 IIRC pop rivets accessed by removing the boot carpet. They will need to be carefully drilled out.
On replacement you can use rivets or I used stainless nuts and bolts with “penny” washers. Treat the area of the holes with rust protection before replacement.
NC owners be aware that front and rear bumper removal will reveal two areas where the chassis rails end that need treatment. I am not sure how many garages will cover those areas.
Fully agree…
RobSteDav:
Have you started on this job yet? I live in Northwich, not too far from your turf by the sounds of it.
I had mine (2013 2L Sport Nav, bought last December) done a couple of weeks ago at “Ring of Bells” garage, Stretton near Warrington. They use black Waxoyl and do a lot of this sort of work and have experience with MX5s. Doughy is very enthusiastic about his work. They also did my 1992 Toyota Hiace MPV a week or so after. It’s far easier to see under that how well the job has been done. I am happy with their work and it wasn’t expensive. They pressure wash first then allow time to dry, clean up where necessary with a needle gun, then spray and inject sills etc. They also cut out and weld if required (a bit of that was done on the Hiace).
Crawling around under cars is getting a bit old for me now (I’ve been doing it for about 45 years), so it worked for me.
Hope this helps.
I too am a big fan of Bilt Hamber.
If the OP gets the metal really (and I mean really) clean with rust converter then their zinc rich primer is lovely. A thin and wholly unsatisfactory silver lining to lockdown 1 was I spent every evening last spring striping, removing every bit of pitted rust in the ‘suds engine bay with Deox gel and then a couple of coats of Electrox before primer.
I’ve always thought hydrate 80 is a risk as it may not convert the rust particles in the bottom of pits. That said I have used it.
I should also add proper degreaser is a must if many of the converter stripper or surface coatings applications from many suppliers are going to work. They say this in their technical notes and in my experience they are right.
I did my heat shield last year, took it off to treat the boot floor. I used hydrate 80 then painted with heat proof aluminium paint on the shield.
Nice job 
I also live in Northwich…
I will give Ring of Bells a bell. I know where they are- on the road past Antrobus somewhere near the M56 roundabout. I really want to get my new ND undersealed before it is really used much as it will be easier.
Thanks,
Ian
Seriously?
You can’t do it whilst it’s in warranty anyway. Mazda will reject any claims, saying the underseal damaged theirs.
All you need to do is wash the underside after the winter. Pressure washer, snow foam, low profile ramps, jobs a good un.
My 3 year old ND is clean underneath, they don’t rust like the old ones. Only if you were daily driving it all winter on a salty motorway, would I consider underseal.
It’s very unlikely you would need to make a rust claim during the term of the Mazda warranty. The rust that causes long term issues is in the box sections and sills of these cars and unless the car is a really pampered garage queen it will fare no better than previous generations.
I would go ahead and get your new car treated although waxoyl would not be my first choice of treatment.
I would agree 100%, also you can view rust prevention either as a long term investment (if you expect to keep the car up to 10 years or so) or paying it forward (and potentially realising a better return on sale price) because it’s the ‘right thing’.
I had an NB with quarter panel bubbling at 4.5 years and an NC which needed rear arch perforations repaired at 8 years, both my ND’s have been fully treated.
Interesting mix of responses. I will speak to Mazda before I do anything, as I would not want to void the warranty.
I had thought of getting some ramps and wheeling it up so I could jet wash the underside. If you get some regular ramps, will it go on both ends? Obviously one end at a time… I usually wash my car on the drive on a weekend, so I am sure this would give an excuse to be outside longer…
I think that if Mazda give me the ok (in writing) that I can add some underseal, then I may get that done, if not, then I would definitely need to look t some ramps - or a cheap winter car perhaps?
If you rustproof it you won’t be claiming on any corrosion warranty.
A valid point, but I bet Mazda do not get many claims, as it will probably last long enough for the warranty to run out!
It wouldn’t be difficult, it only lasts for 3 yrs!
The car being rust free or the warranty 
I guess the question glosrich raises is are there other warranty claims that could be rejected due to the underseal being applied.
Not sure what that could be but worth considering.
Agreed corrosion won’t be an issue inside 3 years anyway.
Personally for me I would get it done, my NC is done. However I would make sure it is a spot on 100% tidy job without any overspray onto any other components.