Yep, Mazda aren't known for their adequate rustroofing of cars during manufacture, they've always been bad for rot and they're still all the same even now. Last year my lad told me he was off to look at an 18 month old Mazda CX something or other at a Honda dealership. I was going off on my hols so couldnt go with him but I told him to look underneath it and he may get a shock, he kind of told me i was an idiot - "it's an 18 month old car for gawd's sake" he said . I just said "ok".
A few days into our hols I got a text on my phone, " You weren't jokiing were you? " He'd taken my advice and looked underneath it, and as predicted he was shocked at the state of it for such a young car. He ended up buying a Honda CRV that was twice the age with half the rust
I've had 2 NCs - both bought under 2 years old and both with quite a lot of rust underneath requiring wire brushing and treating. My last one was a '94 Eunos - a fresh Japanese import (the Japs don't put salt on their roads during the winter months) which is the only way you'll find an older MX5 without any rust.
My recommendation for rust proofing products would be to go on the Bilt Hamber website, their products are second to none for rust proofng and killing off existing rust - t'ain;t cheap stuff but you get what you pay for. Finally, if you're going to do it, do it before autumn comes in and the air gets damp. Rusproofing a car while it's damp is pointless. https://www.bilthamber.com/