Found a rust spot starting; how to proceed?

I noticed the start of a rust spot on my Miata (it's a 95, mostly used for AutoX) today. It's located just in front of the driver's side rear wheel on an otherwise rust free car. Not super visible yet; just starting to bubble (see pictures below). The surrounding structure and the panels behind the area still seem solid; only the areas with the bubbles feel soft.

From the side:

From Below:

First, how bad is what you see here? Really bad, not bad at all, somewhere in between? I'm pretty sure that this area is structural (I think I read it in one if Keith's books); am I right about that? And finally, where do I go from here? I've never done any bodywork, but if there is something simple(ish) I can do to stop this from getting worse I'm up for trying it...but maybe I just need to take the car to a body shop and get this fixed for real? What would I ask them to do? Are we looking at a $200 fix or a $1K fix?

Sorry for all the questions...I lack knowledge on most subjects, but I severely lack knowledge on bodywork issues.

You don't want to know what is behind that. In my experience that means the only way to fix it is cut everything out and weld in a patch.

paying someone - i'd say you're in the $500+ range. Cut, make a patch panel, clean, weld, prep, paint.

Gotta love undercoating/chip protector for locking in rust and allowing it to spread for a long time before you know it's there.

Best sell it now before it gets worse. How's $750 sound?

The vultures come out quick. :)

It's not structural there, only cosmetic. The structural part is behind that. It'll help you or your body shop if you get a new patch panel to weld in, Evan is spot on that it should be cut out and welded back in. I know Blake over at isellmiataparts.com has patch panels.

Oldtin is probably in the right neighborhood to have that area professionally fixed up.

That's the weak point of the Miata - rust usually starts from the inside there and works out. It's not an undercoating problem. If it's started inside the sill, it's structural and you can feel it in the car. If it's just the surface layer, it's not structural.

I have repaired one car where only the external layer that had rust. In that case, I was able to cut out the top layer and weld in a simple patch made out of a piece of flat steel.

I'd get in there and have a look. If you get it repaired before it spreads, you'll avoid some paint work. Since it's on the black section right now, it's easy to paint the patch. That will make a fairly big difference to the repair costs.

I didn't mean to imply that the undercoating stuff caused the rust, only that it keeps it from being noticeable for a long time, until it breaks through.

That's how my Golf died. The floor was a sandwich of carpet, jute, ferrous Grape-Nuts, and a quarter inch of German undercoating (note: was one of the last built in PA). It looked solid from underneath, but you could push it up easily and it made sounds like a beanbag.

The Miata here clearly is not horribly far gone.

oldtin wrote: paying someone - i'd say you're in the $500+ range. Cut, make a patch panel, clean, weld, prep, paint.

Pretty much. I paid $1500 for mine, but it was absolutely down to the structure and the repaint was done at the same time as my bumper, so they blended a lot of paint. They drilled access holes on both sides (only one was rusted) and flooded both with POR-15 to prevent future issues at my request. We used BTDT Racing's fantastic $100 patch panel, which blew them away fitment-wise; it was perfect.

Don't let it get this bad. Don't buy a car with rust already either.

I have some rust forming in a similar spot on my Accord. Not looking forward to having to repair it.

oldtin wrote: paying someone - i'd say you're in the $500+ range. Cut, make a patch panel, clean, weld, prep, paint.

This is about what I paid to have a similar (if not a bit worse) spot fixed on my Miata. Everything inside the sill was clean (thankfully), so it was just cut, patch, paint.

beans wrote: I have some rust forming in a similar spot on my Accord. Not looking forward to having to repair it.

On the bright side, if you ignore it, in a couple years the problem will go away.

Must you cut and replace?

I have some rust bubbling on my car's rear quarter panel so it isn't structural. I was planning on sanding it and hitting it with some POR15 so it doesn't come back. Will that work for a permanent solution?

In reply to MINIzguy:

This is likely caused by condensation collecting in there, if you don't actually get inside the panel it will just rust again under anything you put on the outside.

I had mine done at a body shop. It ended up being $1000.00, but it was both sides and the rot just started to break through in the wheel arches. They basically replaced the metal from the doors to the back bumper from the ground to about 3 inches above the wheel arch. Also note, the $1000 price did not included paint work because we were painting the entire car the next spring. Yours doesn't look as bad as mine was, but you won't really know until you open it up.

I want to thank everybody in here for the responses. Apparently a lot of body shops don't do rust repairs these days (they just want to do insurance work) but I found a small local shop that does this type of thing. They looked at the car and said I'm in the $300-400 range as long as there isn't anything too scary once they start cutting and they can get all the rust out while staying on the black section. I'm going to call them in a couple weeks once they've finished with their current projects and they'll get me fixed up.

Before/after photos?

Er, well, after photos at least since we've already go the "before" ones.

Knurled wrote:

beans wrote: I have some rust forming in a similar spot on my Accord. Not looking forward to having to repair it.

On the bright side, if you ignore it, in a couple years the problem will go away.

If it pisses me off enough it may go away within a couple of months. This car needs about every body panel replaced and I've been looking at ATL CL a LOT lately.

Put the wire brush on the angle grinder and see how far down the rabbit hole goes. Keep going until there's no more rust. Its rusting from the inside of the panel so you're going to need a patch almost for certain. If its really bad you'll have to put the cutting wheel on the angle grinder. That panel isnt structural, but everything behind it is and thats also probably rusting and needs to be repaired. Catch it before it spreads forward of that body seam and the repair panels are only $50 a side from rockauto. If it gets forward of the seam the repair panels are like $150 a side from some specialty miata place that only sells the repair panels. If it is only the outer panel you can get away with using OEM ford/audi/honda chassis weld epoxy to make the repair.

Thanks again for everybody's help and advice on this. It's nice to know what I'm talking about when I take my car to the shop and the knowledge that you guys provided allowed me to do this. The car is all fixed up now...luckily the rust was only on the one panel. It was $370 at a small body shop here in CT to get it fixed up. As requested here are some "after" pictures.

The metal that got cut out:

All patched up now:

Happy Miata:

Well done!

I think if buy a small welder and some scrap before I paid a shop but if you care about the car and haven't the time, you may be money ahead paying the shop like the op did.

If you're happy with the cost and the result, money well spent.

Just a hint. Make sure that the 2 drain hole that are in the pinched seam are free and clear. Run a wire, less than coat hanger size up into the rocker.

Moisture can get trapped in the rocker panel and cause that type rust.

I bought a 90 Miata that had so much water trapped in the sills that it sloshed when you started and stopped.

Surprised no one mentioned the drains at the pinch seam, but last post got it. When I bought my 94 there was a sloshing sound that drove me crazy. Finally found the drains clogged. Seemed fine at the time but it was starting to rust like the OP when I sold it 3 years later.

Stand in front of the rust spot when the prospective buyer inspects it.