Have you tried the guys at Select yet?
Hi Dave, sorry, yes i did. They dont do it unfortunately 
I am also planning to do this - and I am in a worse situation.
Does anyone know someone who rents a lift in Cambridgeshire?
I would only trust under seal applied at the time of construction. Any other time you are dependant on too many variants. Any moisture, contaminants etc will be trapped and you won’t know a thing about it until it’s far too late.
I’ve always used Waxoil after removing anything that can be removed and treating with some sort or rust conversion treatment.
I think is better to use a flexible covering. Yes it wears, but it should be topped up every year, a sort of rusty MOT.
Unless the pro you use has a rock solid reputation this is area that is best left to ourselves. I can’t think of much else so open to abuse or what used to be called a Friday afternoon job:)
So just leave it and rinse it? I’ve just bought a Hozelock lawn sprinkler to rinse off the underside as an interim measure, but I am now worried that if I rinse underneath the car and then it freezes, could this cause damage?! This is doing my head in lol!
You are over thinking this, try and relax, it is not going to dissolve in front of you.
You have made a start so slowly plan your next move. Remember there is no right or wrong and any action is better than no action.
If you get stressed just go for a drive and it will all drift away.
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As others have said it sounds like you’re over thinking things a little. Keep it clean is always good advice. Particularly around the wheel arches etc where there’s dirt traps.
Whilst Mazdas aren’t the best at being rot resistant you can expect a good few years before it gets crusty. The first signs will be the black pain on the subframes etc starting to lift and become green.
I use bilt hamber products as they are easily DIYable if you can jack the car up, they form a nice wax coating which will need topping up every few years but they stay liquid and are generally quite transparent so you would see any issues before they get crusty.
Very effective.
They do UB which is for underbody and s50 for cavities. No special tools required.
I wouldn’t remove any factory coatings unless they have failed as they have been applied in a clean environment and will have bonded to the car well. Wax type products offer a next step of protection.
To give nuts, bolts and steering components etc some protection I can recommend “tec cote acs”
Or ACF 50. It’s used in the motorcycle industry to keep them corrosion free and can be used on most things. It creates a light transparent coating which needs to be reapplied once every 6 months or so, you could get away with annually on car parts I’m sure.
If you want to go nuts and repaint any bits I can recommend epoxy mastic paints. Strip your area to clean bare metal and use a zinc rich primer topped with epoxy mastic, it leaves a good resistant finish and is very forgiving applied by brush or spray. I have used this approach on my classic vws in the past when I’ve restored them and it’s been great.
More importantly enjoy your car
Thanks for the detailed post. One of the reasons I’m looking for someone else to do it is because at best, I can only jack my car up. I havent got axle stands and would probably put them in the wrong place. I have seen some ‘plastic wedges’ that I can put the car on. Would these be any good and enable me to do it myself?
To be honest unless you were reasonably familiar with the topic I would strongly suggest getting someone else to do it and for that someone to be preferably based on multiple personal recommendations, reviews/feedback.
If you’re reasonably competent it can be done on a drive a bit at a time, it should take several sessions over a number of days doing it this way and you must have appropriate lifting and support tools.
Underseal is mostly used to hide rust nowadays. Once applied it’ll generally allow salty water to get between itself and the metal and hold it there. Not an ideal scenario.
Everything underneath needs to go back to bare metal before treatment with at least three different types of modern rust proofing materials and new seam sealant to deal with the corrosion caused by dis-similar metals touching each other. Each of these materials needs to cure before the next goes on. The job takes 4 to 7 days depending on how many ramps the business has. It uses £500-£600 worth of materials. Add for labour and overhead and for any rust repairs needed.
I think taking a 21 plate car back to bare metal is a little bit over the top, the OP was a little nervous to start with so hope he is OK after reading this post!!! 
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You’d leave the rust in place?
It’s a joke, but if you want to be pedantic then as it is a 21 plate the likelihood of rust is very low.
I agree that it would be reasonable to expect that.
I bought one of these. Not tried it yet but might do the trick.https://www.ebay.co.uk/itm/374043697332?chn=ps&norover=1&mkevt=1&mkrid=710-153316-527457-8&mkcid=2&itemid=374043697332&targetid=4585375812880027&device=c&mktype=&googleloc=&poi=&campaignid=554609236&mkgroupid=1308419941753372&rlsatarget=pla-4585375812880027&abcId=9320693&merchantid=87779&msclkid=fca473a0a9481b197dee1611ba390643
Thanks for the replies all. Is it peoples views that underseal is no good and allows water in? I’ve booked my car in for a full underseal with an official dinitrol workshop (Station Garage, Whitland) but dont want to spend £1000 if it’s going to trap moisture…
I just don’t buy that water gets sealed in. If it’s done on a dry day above freezing point with good air circulation it should be fine.
This whole topic is hugely controversial as you know but I would try and steer (sorry pun) a middle course. A conservative approach would be regular washing of muck off the underside and avoiding wet days in the winter. A middle approach would be to get it undersealed with Dinitrol or Waxoyl and that would include removing the arch covers etc. A radical whole hog approach is to strip the bottom of the car, removing even the bumper and then getting everything down to bare metal and applying multiple coats of expensive treatments in a process lasting around a week.
Take your pick but I would just point out these are MX5s not rare Porsche 911 993 Turbos we are talking about…there has to be a limit on what is a reasonable outlay on a car that was designed to be fun, reliable and affordable.
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Cheers. Food for thought…
Just be aware that all MX5s tend to rust from the inside out so it is very important that all box sections are treated on the INSIDE. There are plenty of access points so it is not a problem to do but it does involve time.
One area that sometimes get overlooked is the areas that are accessible when you remove all the boot trim. Getting plenty of your chosen ‘product’ into every nook and cranny in there will be well rewarded in 10 years time.
Another point worthy of note is that it would be wise to do a check and top up on a regular basis.
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The very best thing you can do is not to take the car out on salted roads. Those Eunos Roadsters mostly stayed rust-free until they arrived in the UK. Besides speeding up galvanic action, salt attracts and holds moisture.