Please take a look at these pics and give me an idea of the best course of action that you would take.
The 1st pic is on the inside of the rear passenger fender (towards the rear of the rear :/ )
I hear cutting and welding can result in future rust. I hear bondo is too cheap and won't last. The rears


These are the passenger side rear
will get mud/stone gaurds as soon as I know the corrosion is ok. I am prepping the car for wrap. The rest of the body is good on corrosion except the folds for jacking the car up which have the unusual corrosion.

 

Driver side rear

 

Finally the driver side rear fender (front part of lip)

 

Cut and weld is the best way to go. You just gotta find a nice shell to cut up, or somebody fancy with metal. Resto guys deal with way worse. You should probably rise and neutralize the underside of the car and make sure you find and note all the trouble spots.

 

or buy some new sheetmetal from Mazda...

 

Yes, I agree with the above post. Unfortunately a beat up shell is 3,000 grand if you're lucky so that's out. I know on ebay there is some chopped up panels from japan, rear quarter panels are one of them. I'd buy one of those for 600 bucks and cut the metal out and weld it. Not sure if you know how to weld but I recommend a good (Expensive) TIG welder. Those cheap MIG welders will burn right through. Also to answer your question about if welding will create more rust, the answer is no if you use welder's paint. Also I don't care if you're the best fabricator in the world! You need at least some bondo to make a perfect smooth surface after welding, ask any GOOD body man. It's very little bondo obviously but they use it!

Also for someone to reshape metal to the exact shape is a very HIGH skill to have, I've never seen it in person but boy talk about measuring twice and cut once, not to mention the curves you have to replicate... oooof!

I hope I gave you a good start.

 

So what I did was get rid of the rust and used bondo on the wheel well. As for underneath the rear panels I cut out the rust, treated it, and will get around to it in the future. Your not able to see it and I will be installing the mud/rock guards. I wrapped the car so it is able to be removed later down the road and weld in metal if needed. I don't think the rust was as bad as I had originally though.
I have little spots of surface corrosion here and there that I sanded and treated. I will probably have the sub frames painted when I do the rebuild on engine and rebuild the diff.
The important thing to me is it is preserved and 2nd it looks cool. So far so good.

My 1st time working with bondo. It was interesting to say the least!

 

BTW that bondo is serious stuff. I never knew how strong it was once cured. No idea how long it will last, but I don't see how it wouldn't for a few years. The car is garaged.

 

 

I hate to say it but that's not going to last very long, 1 year max. It's recommended that you only go 1/4 inch with bondo and looks like you exceeded that.

Anyways, if you're going to go the "hack" way, here's how to do it. (This is not the correct way but will last much long)

1. Get long hair fiberglass (Any parts store sell it)
2. Slap it on and make a good "frame" for the bondo to hold on to (Try and use as little as possible to make the shape you desire, it takes much longer to sand down)
3. Once's satisficed with the shape of the fiberglass, slap some bondo on.

This will make a much longer repair because bondo isn't meant to "structure" only shape. Fiberglass is basically a better base to work off of, instead of straight bondo.
Also some people use chicken wire for structure integrity but I personally never used it.



Or... you don't have to listen to me and experiment and see what happens. Live and learn I guess but you're wasting your time.

 

Well it is way to late now. This was done awhile ago. It is no where near 1/4" thick though. It was a small hole after I removed all the rust. It was treated. Then I used a mesh and put a decent amount of bondo, but sanded it down to almost nothing. The mesh provided the structure.When I say nothing, I mean no where near 1/4". The amount of bondo you see is no where near the size of the hole. It is just to make it as smooth as possible.
Live and learn like you said. I never messed with the stuff before..... like at all. I have never done any body work. I would be surprised if it doesn't last more than a year, but again we will see. Again it isnmyb1st time and done before I got any advice. I am pleased, but yes I hope it lasts.

 

Creepin.... thanks btw. I will update if anything odd happens. When I got rid of all the rust and then some there was still a hole. It was never all cut out as it didn't require it. I also could inspect the metal "fold" and all looked nice and clean up underneath and inside. I am not trying to put a bandaid. I did want a fix, but such a small piece to cut out a huge chunk seemed wsy over the top.
As for the lower out of sight sections. I am not sure how that will work either. Like said before, I just cut out all the corrosion, treated the area, and painted it.

The rest of the car looks pretty good imo. Just those spots, and a little surface corrosion here and there.

 

These are terrible pictures for the light, but if you can see it the hole was a little less than doubled, nice shiny metal, the treated with a converter, a heavy metal mesh attached, then bondo applied .

 

The fiberglass and bondo will work great for the outside, I agree with you and it's over the top for those areas. But for the areas under the car it's a different story. That's where I'd try and find a chopped up FD and purchase the areas you need. If money is an issue, then yes, take as much rust off as possible and use a rust converter paint after getting the lose rust off. Now, it a pain in the *** to get into areas that are under the car. I highly recommend a hand held bead blaster. It's dear near impossible to get a grinding wheel in all the nooks and crannies. If you don't have the patients for all this, then at least use the rust converter. Body work is expensive in shops for a reason. It's time consuming!

I recommend Eastwood brands. (There products are super great and have a whole section on there website of just metal restoring products.) The rust converter is great because it can transform light rust into a paintable surface that won't rust again because it chemically changed the rust.

 

Thx again. I actually have east wood, rustoleum rust dissolve, and another.. I hope it lasts awhile. Again I just want to stop and preserve. Of course I also want it to look good!