Sealing and Touching Up the Factory (OEM) Undercoat on the Undercarriage

Asked by kimcane Nov 01, 2023 at 10:07 AM about the 2021 Mazda MX-5 Miata RF Grand Touring RWD

Question type: Maintenance & Repair

On the ND2 MX5, Mazda installs from the factory a rubberized coating that
starts after the frame rail and pinch weld and stops just before the front and
rear crossmember and transmission tunnel. I noticed that 2 very small
sections of the (OEM) factory-installed rubberized coating has peeled. I am
also aware that Zeibart rubberized coatings and rattle-can rubberized
coatings will trap moisture and cause accelerated rusting. However, I do not
want to leave these 2 small areas of the metal exposed. The 2 tears in the
undercoating are about the size of a nickel. There are no dents. Would
applying a little bit of automotive primer followed by applying Rust-Oleum
Flat Black Paint be effective? Or, should I apply the primer, add a little bit of
Bondo to match the depth of the OEM coating and then apply Rust-Oleum
automotive flat black paint? I am also open to other strategies. Please
advise. 

3 Answers
307,255

Gene answered 5 months ago

The truck with undercoating is to throughly warm the vehicle like in a paint booth, this takes care of trapped moisture and helps with adhesion of the undercoat. On spot repairs I have used hot air guns and even high intensity lights to warm and dry the area. Good luck.

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38,925

beatupchevy answered 5 months ago

Clean the lose rust , hit it with rust reformer , Work it into the metal while wet with a wire brush , Prime and paint or just paint it . Wipe clean with thinner and light buff with a brass brush between coats . That's about the best I've come up with .

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202,415

Ed92626 answered 5 months ago

If you're under warranty, have the dealer fix this. Otherwise, This is the underside of the car, not a "show" finish. If you have to repair it, peel the coating back as to its loose limit and trim off. Apply primer and don't worry about the depth. Then apply any rubberized seal. I've even seen "Flex Seal" spray used effectively for this. Just apply until the surface is smooth.

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