ND4 Underseal Needed?

  1. My model of MX-5 is: ND4 hopefully
  2. I’m based near: Kent
  3. I’m looking for technical help or recommendations on: Underseal

Morning All,
I’m looking at getting a 2019 ND4 and have read some stuff about older ones needing underseal protection as well as the newer ones. The plan is to keep the car long term so is it something worth getting done asap on an ND4?


If you are keeping it long term and using it through the winter, then yes get it done.

If it’s summer only use, it will be ok for a long time, they have wheel arch liners and are partially undersealed.

The factory underseal isn’t sufficient for winter use in the UK.

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The Mk4 ND is constructed from high tensile steel, which by its very nature is resistant to corrosion so rust should not be such an issue from a structural perspective, however although the bonnet, boot lid and front wings are aluminium the other cosmetic panels are still mild steel. So the rear quarter panels may rust in time. Aluminium has been adopted for a number of the suspension parts.

Time will tell if serious structural corrosion will be a problem, but as said if the car is a keeper spending £400 getting the cavities treated and a general underseal is a good insurance


Good to know. The idea is for a weekender that will get used on the odd occasion so it’s probably worth getting done.


If you want to protect your Mx5 , whatever Mk it is it’s a no brainer to get it treated professionally as Mazda never did.
ND’s are already starting to rust.


Get it done. From what I hear, early NDs are already coming in for rust repairs and appears to be little better than earlier versions. Make sure it gets done from the inside - not just a coat of underseal underneath. They rust from the inside out.

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I’m assuming i’ll find some recommendations in Kent somewhere on here?


Have we got any pictures of rusty NDs?

The suspension components and subframes will rust, but I would think the body would be resistant unless really abused and let go.

Where did you hear that? Seems highly unlikely.

My thoughts too… Most car subframes and suspension components rust and it is not unique to Mazda… It’s a fact that new cars are manufactured to a price and whatever make of car you buy, there will be areas where quality is questionable…


At the end of the day they only need them to make it through the anti-corrosion warranty period.
Even then it’s hard to claim without full Mazda service history, they always find an excuse!

Yes. But don’t just have underseal put on.

In a perfect world you would wire wheel off the existing rust (yes, there will be some), then
Treat with Bilt Hammer Hydrate 80. Two coats. Two days curing time.
Paint with Corralles glass reinforced paint. Two coats. Overnight drying time.
Underseal with Raptor bed liner (which can be colour coded).
Remove interior.
Treat all cavities with Bilt Hamber cavity protection wax.
Ensure all drain holes are unclogged.

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