How best to underseal

  1. My model of MX-5 is: 54 plate, mark 2.5
  2. I’m based near: __Chester
  3. I’m looking for technical help or recommendations on: __undersealing

Hello all, my MX5 has just passed its latest MOT but there was a list of advisories concerning the suspension arms saying ‘corroded but not seriously weakened’. I have spoken to a number of local garages (and indeed two have inspected the underside) and all report that, yes, there is surface rust and corrosion and none of the arms will ‘shatter like glass’ on the next small bump in the road as Mr MOT inspector would have me believe. Anyway, felt it was time to ‘tidy up’ underneath with a view to applying some good, reliable underseal product. Looking for pointers as to which stuff to use, please.
Looking to do this in the main, myself using car ramps and/or axle stands and plan to largely brush the stuff on, with the option to spray the more difficult areas.
Hope you can help. Thank you

Plenty of options out there! You will need a good rust treatment/converter before any under seal type stuff is applied. I used Bilt Hamber products on my NC2 and it did a great job :+1: I bought a set of wire brushes for the battery drill and also had a hand wire brush. Every bit of loose rust etc needs removing before applying rust converter, then, when that’s reacted, you can apply the protector. It’s not a hard job in itself, more awkward really. I used axle stands. I was changing out the suspension so got decent access inside the arches and to the suspension components while doing the suspension. Worth doing definitely and after reading horror stories of some garages making a right mess and not doing a decent job, worth having a go yourself. It is a messy job though! Bought my stuff from Opie Oils.


as an mot tester it really frustrates me when other testers use the word corrosion! they should just use a manual advisory and put surface rust, i mean as soon as you have corrosion on the mot history im sure it will devalue the car a bit.

the amount of cars i see with this done is unreal and i always put a manual on there to say that it isnt corrosion, but just surface rust.

good luck with the job in hand mate.


Totally agree!


Hi Barrie, thanks for your reply. There is clearly a heck of lot of different rust ‘remedies’ out there! You’ve got your rust removers, rust preventers, rust beaters, rust convertors - which to go for??!! Exactly which did you use? In terms of the protector, people speak very highly of Hammerite’s Underbody seal (with added waxoyl) - which you can get in brush-on and aerosol form - so am tempted to go for that.

I used Bilt Hamber Hydrate-80 for the rust converter, TBH, I’ve previously used Hammerite rust converter and it looks almost identical? :thinking:
For the under seal I used Bilt Hamber DYNAX S-50 extreme. Both in spray cans with a not too wide spray fan. Gives you a bit of control where you put it! :grin: the S-50 comes with extension nozzles to get inside cavities. All good products IMO.

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It’s one of those jobs you can do in as much detail or as little as suits you in terms of budget, preference and how long you want to keep the car.

Generally I’m not a fan of just spraying black sticky underseal everywhere, it looks like a bodge job to hide big problems and you can’t clean it.

If it’s only surface rust, work in sections, pressure wash it off in the summer, wire wool or brush it, rust convertor, then spray neatly with black hammerite or similar.

It will look like the original finish.

Thank you for your input, glosrich, and you are quite right with your opening paragraph! I, too, have seen some recent pics of a garage spraying it everywhere and it slightly horrified me! You must work methodically (which comes naturally to me!) and I think I favour brushing it onto the ‘easier’ exposed surfaces and maybe spray into other awkward areas. Am now torn between this Bilt Hamber Dynax S-50 stuff or the Hammerite underbody seal (with waxoyl). I am told that the latter has a dark brown finish as opposed to black though.

The Bilt Hamber stuff is good. Use their rust convertor first.

I need to correct my last post - the Dynax S finishes dark brown. The Hammerite is black.

I used CORTANIN F Anti Rust Converter on my Mk2.5 after getting the same advisory last year. Also done the rear subframe. Picked it up off ebay & it worked well. Bit messy though as you brush it on.
Sprayed it with an underseal from Lidls that was on offer at £2.99 a can. It done great job covering and no nozzle blockages!
Just a tip but probably worth waiting till the spring and any salt on the road is gone. Give the underside a good hose down with cold water and allow to dry first just to get rid of any salt.

Over the last weekend I managed to prep 3 of the four arches (not fully ready yet) but would like to establish best way to clean the rusty areas prior to the application of the rust convertor once I have wire brushed and emery papered as best as possible. My goto product would be white spirit. As we speak I don’t know exactly the condition of the underside, but in any event is it necessary to remove the exhaust sections or just mask them up? Another thought - no need to apply the underseal to the rigid plastic arch liners…correct?

You need to take the wheel arch liners out to see where any potential rust is - esp the rear wheel carpet-like ones as the leading edge is where it usually starts and you need to get access to inside not just clean, anti-rust and blast the visible bits on the underbody. The dangerous rust STARTS inside - esp the panels!

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Try Mark Evans on Facebook at Penryhn Service Station. He’s North Wales so local to you

It sounds like you’ve just got surface rust. As others have said a good pressure wash of your wheel arches let it dry and follow up with a wire brush.

Your suspension arms etc I’d just clean up as best you and touch up the paint and follow with Bilt hamber UB.

If you want to be thorough and start stripping things back I’d clean back to bare metal and coat with an epoxy mastic paint. For the underbody itself you can touch up with stone chip paint for a factory finish.

If avoid the black shutz type under seals as they look a mess and don’t last long before trapping moisture and causing more rust.

The bilt hamber stuff is great value and diy friendly

Interesting… give me an example of such seals, if you would, please

There’s loads, they are usually the generic ones labelled as “under seal” hammerite, tetra are the most common brand.

The problem with them is they tend to dry out crack and let in moisture and it’s difficult to see when it’s failed. It’s also a pain to clean off and reapply every couple of years if you want to use it properly.

There’s better products now like some of the dinitrol waxes or bilt hamber which stay flexible and can be more easily recoated.

When I’ve restored cars in the past Where I’ve gone back to bare metal I tend to just use an epoxy or zinc primer followed by stone chip or raptor liner followed by body colour if I’m going to that effort.

One thing I would like to try on a factory fresh or newly restored car is to apply some ACF50 anti corrosion spray. It’s a clear spray that I’ve used on motorbikes to ward of corrosion and it seems to work well and is invisible once applied.

I don’t know how well it would fair under a car unless it’s reapplied every six months or so which I couldn’t be bothered with.

On a well used car, one that’s already been under sealed or one that’s not being restored I’ll tend to clean back whatever is on it to check for nasties and tidy it up and follow with one of the above mentioned wax coatings and I’ll do the cavities at the same time.

Just my opinion but it’s worked for me on various cars made out of the 70s finest steel :joy:

‘Just my opinion’ Montana writes - well my opinion is that you are best off letting the professionals do it. Cleaning underneath a car, treating rust, and finally undersealing, is a foul, disgusting and messy job, and not one I would even contemplate - especially at 66 years old !

Just my opinion !!

If I were to do it, I would want to have the use of a hoist for a day. Even then it seems like a nasty job.

ACF50 following a pressure wash every October would seem a reasonable idea.

Less necessary in the Summer months with no salt around…