In reply to NOHOME :
At some point, there will be hidden welds somewhere. And while I'm calling this a "restoration"- it's not about 100% originality, but getting back a solid car that will last another couple of decades.
I appreciate the thought of replacing the whole panel, but even if I did one large chunk of the fender, it would not be the entire part. I do plan on finding the tiny holes where I can put some protectant back there. And all of the structural parts will get something on the front and back before I put it back together- there is enough space for that.
And this is what I bought back in 2014- sort of (no primer on them). https://raybuck.com/product/1999-05-mazda-miata-upper-rear-wheel-arch-driver-side/?gclid=EAIaIQobChMI4JuXxNWP3QIVWrjACh2RAwGkEAQYASABEgIkKvD_BwE And back then they were considerably less expensive. Not sure why.
Some more work...
so I figured there were 4 patches to be made.
The first in the wheel well.
The second the hole in one of the pictures
Both were tougher than I thought as the original metal was thinner than I thought so I fought with weld blow through. But got it done primed and painted. This area was originally just treated, I just wanted more protection.
Still have two more patches before I move on.
While I was cleaning up, I took a quick look at the rest of the rear fender rust- looks like it's mostly really bad outer fender rust, so I'll have a lot of clean up, but less patching up higher, I hope. Once I get the other two lower patches in, I'll take more fender out to see other rust.
And when I get more out, I'll be able to get to the back side of what I just finished, and put something down there to reduce the speed of new rust. Hopefully, the time I become the dreaded PO, the car will be way older, and it will have survived 20ish more years. Gentler years, as I hope to drive a lot less in the snow.
Just one part welded in. This took a long time to get fitted, but it's more structural, so getting it in cleanly was important. 
Finished the middle section on the bottom. 
Its primed and painted for now
Then cut out the next bad skin. 
I was hoping that the tabs that the body is welded to would be ok, but many just pulled out with the damaged metal. Oh well. Could be worse, and this is a lot easier to fabricate.
It's been a while since I worked on the car, but the current forced vacation gave me a lot of free time.
So yesterday, I cut the patch panel to make it fit pretty well..,
Today I cut out the body and started to trim it to be as good as I can with a cheap panel.
On the upper right, I found some more crappy metal that has to be replaced, but given the usage of the car, it's about getting it to fit really well, and with what I have- prevent the rust coming back the best I can. It helps that I don't plan on commuting on salty roads any more, so...
A very curious find, though. While I was sanding the paint to get to bare metal on the body- I found quite a bit of bondo. And under that, some red paint. WTF??? I can remember getting hit in the rear of the car that needed fixed, and I had the salt blasted paint in the front redone once. But I very much don't remember fixing anything in this area. Ever. This was up in the area of the door handle area- so it's not like I curbed the car.
There wasn't really any evidence that there was even a dent there- just a low spot that someone filled over the original paint.
The PO was Mazda- since I've had the car since new (like under 50 miles on the car). Perhaps there was an incident in the lot?? Possible, I suppose.
Just odd.
Some more progress today, cut out the rearmost rust and preped a part of the replacement panel. 
And then primed and painted the metal under. I noticed some soft material between the outer and inner fender. Since I don't have any, I painted in some bedliner which is kind of soft. 
I'm going to try to stich both panels in tomorrow.
Once this side is done, I'll move over to the drivers side and work on it- which is a little worse than the passenger side.
With all of the time under the car, it's interesting to see all of the rust on the suspension. I'll take it all apart and clean it all up. New springs, sway bars, and shocks front and back because they are pretty destroyed. That's what you get commuting every day including when the roads are coated with salt.
Progress was slowed at first when I had emptied the gas bottle for my MIG welder.
Then it stopped when I ran out of wire.. More shopping tomorrow.
Need to also get some better sanding materials.
And now the passenger side metal is done!
It's not perfect by any means- the patch panel I had was kind of twisted, which made fitting a big pain. And my choice of doing the lap style didn't work out all that well- now I have a small channel to fill with good filler. Not deep, and not violating any filler guidelines- but it's not what I wanted to do.
One big lesson that I learned was doing smaller patch pieces instead of one large piece. The panel I got isn't straight, and isn't' finished like it came from Mazda, so there was a lot of "forcing" to get it to line up well. Which has lead to some high and low spots in the middle of the panel.
And that will work out well for the drivers side. I'll take pictures when I get to it- but the damage is a little worse in places, and more spaced out in others. As I see it, there are 3 distinct places that I can make smaller panels. And then where there's some surface damage, I can use some of my trick paint behind the panel where I can.
Even though it's not what I was hoping for, I also think it turned out pretty well considering that was only the second time I did any significant body work. And this one was more rust and whatnot.
Looks good! A little body filler, and Bob's your uncle.
Long breaks in work. With some time on my hands, decided to start on the drivers side.
It looks worse on the outside than the passenger side, but under the structure is better. 
The hole, I punched, as I was using a air chisel to clean up the rusted outer skin. Easy patch.
Both patch panels are cut and lined up, and I'll try to melt them in tomorrow after the primer dries.
Having done both sides- I think Mazda missed some protection- there's *something* between the outer skin and the under structure, but it's really minor. So in the places I'm patching up, I'm putting in some bed liner as a separator.
The black stuff is Hirsch's Miracle paint. Just a rust converter I found in Hemmings many years ago. Seemed to work before, and I can paint over that.
Once these two holes are melted in, I'll go after the sill, which has a lot more damage. And if it's like the passenger side, I get to fabricate some inner rocker structure.
Would like to get the body done this summer, so then I can focus on the engine and suspension.
Fun-ish side note. Part of the reason I have time is the fact that I'm working from home full time now. Mostly from Covid, some because the organization is doing odd things. But I also got some nice masks at work when I picked up all my stuff. These are WAY better than the hard dust masks we get at HF. So it's a lot nicer to do this rusty, dusty work.
So, what's the engine / suspension plan?
Since this is our retirement car, just get it back to stock. The #1 thing is to replace the PCV system and the valve cover gasket, then normal maintenance, Probably an NB2 header, cat- and then a FM stainless exhaust.
Suspension- FM mild upgrade.
It's not the car we would race, just one we will enjoy, so it needs to stay comfortable- it handles so well as it is, not much needs to happen.
The key thing is that the car needs to last as long as possible. It and our GTV are two cars we plan on keeping forever.
Cool, I'm recognizing I'm OLD, in that I'm actually successful at keeping stuff stock. Or maybe it's that I'm poor.
Carry on.
