The issue with repairing is that the rust is often just as bad on the inner skin of the rear arch, meaning that really you need to cut the outer arch out to get at it. Do a google search for MX-5 rear arch rust and have a look at some of the photos when the outer skin is cut off, and you will see what I mean.

No point cleaning up just the outer if it's getting bad, it will literally come back again, like it already has.

 

If front then replace. If rear then repair by getting the cut out and new ones welded in properly across the full length of the arch, not just at odd patches where rust is showing. It hangs out in the seam and will shortly return. Quality arch work is expensive so be warned.

 

If the lip of the rear wheelarch is rusty then you will want to take a very close look if there is more rust hiding in the seams at the front of the wheelarch.

If you look at this image its the seam that runs along the top of the new black panel.

 

If the lip of the rear wheelarch is rusty then you will want to take a very close look if there is more rust hiding in the seams at the front of the wheelarch.

If you look at this image its the seam that runs along the top of the new black panel.

Yes, that is where the rusts starts. It comes from the back, so patching it at the front is a bit of a useless fix, because you do not fix it by the source.

Best way to fix rusted arches is to cut away the side / sill and replace it. The sooner you do, the less rust you have to fight below the skin.

Have a look at my thread to get an idea how bad yours could look from the inside....:
http://www.mx5nutz.com/forum/index.php?showtopic=337441

 

The other thought is how good the last repair job was. If it wasn't prepped and painted property rust will come through pretty quickly.

 

Thank you everyone for your replies.
The rust on the rear wheel arches is in different places than the previous repairs about five years ago which are still good. I will get the repairs done asap.
Again, thank you.

 

The other thought is how good the last repair job was. If it wasn't prepped and painted property rust will come through pretty quickly.

They rust from the inside out, there is no way to prep the metal effectively after welding it in. You can only try and soak everything in protectant every year and hope for the best.