DIY/WIP repair rear wheel arch rust -

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Old 02-11-2018, 10:37 PM   #1
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DIY/WIP repair rear wheel arch rust


So this isn't a DIY because I know what I'm doing, it's a DIY because I'm determined to fix it and may or may not succeed.

Sitrep: 14 years old with occasional winter use. Over the last year a couple of bubbles showed up in the lip of the right rear fender. Majority of the rust is where the aero skirt goes over the wheel arch lip (I suppose this area accumulates moisture).

Further inspection reveals rust is all around the wheel arch BETWEEN the outer and inner sheets of metal (also wtf is with this design, it's obviously going to rust). I can remove bubbles all day, but that metal is impossible to get to.

For the last 4 years both wheel arches have been stuffed with some kind of heavy grease, which I suppose is why the rust has been contained.

Plan of action:
  • Clean up surface rust
  • treat, primer and paint reachable rust spots
  • apply hydrophobic coating all around both wheel arches
  • reseal the rusting seam with whatever is best

Pictures of work in progress below, could use advice on how to proceed from here.

Initial situation. The black stuff is whatever paint I tried to temporarily cover up the obvious rust with:

Wheel well liner removed and cleanup started. The worst area is at the bottom, and it's hard to get maneuver in there. Clean up tool is a Dremel with a collection of wire wheels and stripping wheel

Close up of the worst area.

Rust extends through the pinch weld, which is not amazing

Clean-up in progress, view of the inside of the wheel arch. You can see the seam which producing all of this rust. This is definitely a breathing mask and safety goggles situation. There is rust dust EVERYWHERE.

This is what I'm using as a treatment prior to priming. I've tested it on assorted rusted metal and it's pretty magical.

The cause of all this rust. Rust accumulation has expanded the seam and buckled the wheel arch. I can get the loose rust out of it, but I don't see how I would ever clean it all out, so thinking treat and seal from oxygen is the best I can do.

Current situation:

Thoughts? Experiences?

Last edited by Loki; 02-12-2018 at 12:10 AM.
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Old 02-12-2018, 04:36 PM   #2
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Same thing happened to the Protege I had. I tried the same thing you are doing with rust converter and it pretty much did nothing, didn't even seem to slow it down. Not sure if trying to seal it would have helped or not.

When I replaced it I looked at new Mazda 3s and 6s and saw they had the same design and same problem. Only got an RX8 because I moved to FL and don't have to deal with rust anymore. Working on cars without rusty bolts is amazing, sweating buckets while you are doing it sucks though.
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Old 02-16-2018, 01:39 PM   #3
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So the bottom portion of the wheel arch is actually not so bad: the metal is snugly packed together and there is sealant/adhesive between the sheets. While it looked bad, most of the rust is surface pitting. Ground down, treated and filled, no drama.

The mid and upper portions of the arches are worse because something has spread the layers of metal apart. I suspect this was frost heave.

Some of the pockets had perforation through the inner (thinner) metal sheet, but not the visible body part, so I cut those pockets away entirely. There is no evidence of sealant or adhesive in there... soo. thanks Mazda.

For other pockets, I was able to clean out with a cutoff wheel and treat with rust converter, however I can't see how deep they go.

My plan at this point is to apply rust converter and clamp the pockets closed, to force it down whatever invisible rust channels may exist, then re-open, apply adhesive/sealant and reclamp them for good.

Then repack the lip with grease. I feel this helped contain the rust that started before the grease was applied. IF you drive in the winter, grease those arches up.
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Old 02-18-2018, 12:48 AM   #4
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Starting to look like something...

Wheel arch pressed back together..


Make sure there are no gaps in the bondo, as that will make all this work useless. I found a small mirror help me see wtf I'm doing.
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