There are so many different products on the market that we really need a breakdown of what they will be doing and using for the money.

I had my 2011 mx-5 rust treated at about 1 year old.

I've used it in the winter months, but it has been garaged.

Still remarkably rust free underneath. I used a company called Rustmaster, but I see they no longer exist. I guess that means my warranty is void too!

I also took it to a specialist to get the wheel alignment set about 3 months after buying. The NC mx-5s have fully adjustable front and rear suspension - I assume your model does too. It made a great car even better.

A proper job means taking out the arch llners, sill covers, undertrays, cleaning up, removing loose rust, converting surface rust (or bare metalling), applying product including to the cavities and reassembling. The Mk4 isn't too bad generally but the paint on the subframes is thin and that is where I expect you have seen rust.

Mine was pristine underneath when I bought it and I got MX5Restorer at Pevensey to protect it. It didn't need much cleaning beyond a wipe over and they did it with Bilt Hamber clear wax. They charged me about £400 8 years ago. There is still no sign of rust.

With the best will in the world it's not likely to be less than £600, £800 wouldn't be unreasonable but it will depend on how much prep is needed.

If yours is a good one and you intend keeping it, frankly it's well worth doing. Rust repairs are expensive. The satisfaction I get from seeing a completely rust free underside at MoT time convinces me I did the right thing.

Had I sold it after 2 or 3 years it might not have been worth any more but I suspect it would have been easier to sell.

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Edited by Manatee on 01/07/2025 at 22:13

I have since heard stories about all MX 5's being prone to rust because they aren't rust proofed in the factory.

They are indeed rot-boxes. Since their birth in 1989 when they effectively replaced British sports cars, thousands of them have already turned into rust.

At the moment there is some surface rust underneath and as I intend keeping the car I am considering having it professionally rust proofed.

Well, make sure the 'surface rust' is completely eliminated and the surface primed with a rust remover, e.g.Kurust, before the proofing is applied.

I've been quoted between £800 and £1000 and I'm looking for some advice as to whether this is a sound investment or not and how long the treatment lasts for.

Any car with a year's MoT is probably worth at least £1000, so if you prolong its life by more than a year you're ahead - if you intend to keep it beyond its expected life.

My 1980 TR7 which I bought at eleven months old (for £4250) was Ziebarted from new. It has lots of £1 sized yellow grommets covering holes in the bodywork into which the black tarry stuff was sprayed. I have no idea what it cost to do it but it was well worth it. I used it as my daily workhorse through ten salty winters (hopeless in snow). The only time has been off the road was a brief 'SORN' during Covid. I suspect it is one of the few TR7s still around which has never needed 'restoration' ; with original rust-free sills.