Take the door sill protector off (they just pop off with 3-4 clips when you pull them up - no tools required) Then look inside the holes with borascope if it’s small enough.
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I did this, but also found the big hole in the rear wheel arch, which is very handy for getting a cavity wax lance in. Without a borascope ( I have ordered one ) it’s possible to see through the various holes that the sills are internally rust free. Just that annoying bit at the very rear of the sill has rusted, but not structural.
That hole only gets to one inner section of the rear inner sill sections…there are two more sections to also inject cavity wax into.
M-m
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Is there any advantage in removing the seat & carpet to inject from that direction?
Wow - that’s a thorough job!
I have been working on mine a bit at a time & luckily it’s not too bad underneath. I can see places though that would crumble if left unprotected for a couple of UK winters.
I have been recording my progress on Instagram with the account Little_Black_MX5 if anyone is really bored 
I removed the front bumper today to have a proper look inside the chassis rails. Happy to see there is no rust inside them, and the wax that I injected from other directions ( pre bumper removal ) had penetrated well, but I still gave it some more while I had the chance. It’s said that these rust from the inside out, but mine have surface corrosion on the outside, and are totally rust free inside. I have walked away from a few MX5’s because I asumed the visible rust meant the rails need replacing.
Edited to add - I guess they could be rusty in the tiny gap between the inner & outer skins.
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Your last sentence represents most of the issue tbh.
All you can really do is drill a few holes, flood the gap with nasty stuffs, and plug 'em.
Actually, I’d let it breath and not plug.
But you actually spotted the root problem…that moisture gap.
Fair play to you.
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I tried drilling into one of the rails to do just that, but gave-up when I fried a brand new 6mm HSS drill bit. Any advice on the type of drill bit required is very welcome!
Also to note - the hardtop sold on EBay for £665!
That’s a decent chunk of cash off the purchase price of the car, and worth bearing in mind when buying an MX5. Assuming you don’t wish to keep the hardtop, which I definitely didn’t.
Now, what MX5 goodies can I spend that cash on… 
Try a cobalt bit
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Thanks, will do. It’s been years since I tinkered with cars, and new things have been invented in that time. I’ll add cobalt drill bits to a long list of things to buy.
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