Good Morning , I noticed a small dod spot of rust on the rear driver arch from inside as the arch facing the tyre. I have not got rust anywhere else on the car. Then tried to see what’s going on and underneath the arch inside all around I took a lot of mud 
used a brush and more mud come out , but the strange thing was only that side had so much mud. Can this mean that water moist etc… build up and caused that spot of rust ? because I can’t see rust on the car anywhere else. Am not sure what to use to treat it , any ideas please if you could help there are so many products out there not sure what to get 
I had exactly this, noticed it last summer. One spot only, same place.
Removed all the mud from the inner arch- to avoid providing an area likely to contribute to dampness/corrosion.
Then ‘burst the bubble’ of paint and cleaned/sanded the area-smaller than the top of a pencil. Painted it with rust converter, then a spot of primer, then paint.
No further issues, no progression, and have continued to try and keep the inner arches free of mud.
I was hoping it was due to maybe a stone chip hitting the inner arch rim, as opposed to rust from within ( caused by the wet mud stuck in the wheel arch). We live in hope.
Hi David,
Wow … sounds the same issue, thank you for replaying I just found it on amazon I bought it . Hopefully will be ok , I think you right have to keep the wheel arches clean of mud and hope for the best .
Thanks again!
Not sure which MX-5 you have, but on my last NC2 I also had both rear arch liners timmed, so there was a gap between them and the body.
You then reduce the chance of a build up of damp material against the metal. Of course proper cleaning really helps as well. A lot of people just concentrate on the visible bodywork and forget about cleaning the wheelarches properly.
I think you are very right I have the MK3 58 plate but I look after it very well , when I saw the amount of mud come out from the wheel arches I was shocked I never thought such a build up can go there .
Worth at least an annual jacking up of the car from the rear and take both rear wheels off. Only then can you properly clean that area 
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I’m guessing it’s the nearside wheel arch, probably going down muddy lanes picking up the debris from the verges?
The arch liners come off easy enough, clean them down (jet wash) and do the arches whilst off. Treat any rust with something like pictured above and paint some underseal under there before refitting the arch liners.
As for the rust bubble just use a touch up paint (get the Mazda official paint in colour) after first treating with a rust converter, again the stuff pictured above, use a primer then finish in colour, probably more than one coat each.
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Is it the spot welds? They tend to rust as paint ends up thinner around them.
both inner arches with accumulated debris in my case…now trying to clean regularly so as not to allow this to build up-as especially when wet this willprovide a perfect environment for rusting
not in my case, although I get your point