Rear Window Rust Repair

I'll just get right to point. If you really want to do something to help keep your 2002 from turning into a rusting pile of trash, then replace your window seals! New seals may not impress your buddies as much as new wheels or pair of Webers, but your car will thank you. Want to know what happens when window seals are neglected? Read on.

rear window 01

A worn out rear window seal allowed water to leak in, turning part of the window shelf into Swiss cheese.

rear window 01a

The same area, seen from inside the trunk.

Spot Weld Hell

I had no idea this part of the car had a rust problem when I started the restoration. It just goes to show that you never know where rust is going to turn up on a 2002. And for such a small area of rust, this repair has consumed a large amount of time. Between my car and the donor parts, I estimate that I had to drill out more than 100 spot welds!

I began by taking apart a donor rear window section, which had been kindly provided by a local wheeler and dealer of used BMW parts. This gave me the opportunity to understand how things came apart before I tackled the real thing. During this excercise I decided that it made the most sense to remove the lower panel first, but before I could do that I had to sink about 80 bucks into the purchase of an Ingersoll-Rand angle drill. With my electric drill, there was no way to reach all of the spot welds. We all know that this is really just an excuse to buy tools;-)

rear window 02

I began by removing the lower panel. An angle drill is necessary to reach the spot welds above the wheel wells.

rear window 03

There were a bunch of spot welds and many of them were in tight quarters.

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With the lower panel out of the way the true extent of the rust was revealed.

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The rear window structure from a donor car. The donor parts are in the lower part of the picture.

rear window 06

The bracket for the fuel-vapor tank was removed to gain complete access to the repair area. As you can see, the bracket was rusting too.

Out With the Bad

With the lower panel and the bracket for the fuel-vapor tank out of the way, I marked my cut lines and carefully cut as much as I could with the body saw. Instead of drilling out the spot welds along the window frame, I used my angle grinder to grind down the remaining material until it was thin enough to remove by hand. This left the outside of the window frame intact.

rear window 07

Cut lines for the repair were carefully laid out.

rear window 08

A body saw made quick work of the rusted area. I ground down the material I couldn't remove with the saw, and then drilled out a few more spot welds.

rear window 09

The old piece was used to mark the cut lines for the patch.

Instead of welding the window frame, I decided it would be best to use some JB Weld metal epoxy. The outer window metal is pitted in places and generally pretty thin from the manufacturing process and using the MIG welder would have created quite a mess. To improve the chances of getting a strong bond, I drilled a few holes and allowed the JB Weld to ooze through.

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The patch was glued along the window frame with JB Weld metal epoxy.

rear window 10a

When the JB Weld was dry I welded the seem. This picture was taken after the first round of tack welds.

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The worst of the rust was cut out of the lower panel and...

rear window 12

...replaced with donor material.

This is glue... Strong stuff

Taking the "glue instead of weld" idea a step further, I used 3M Panel Bonding Adhesive to replace the lower panel. I had been waiting for a reason to try this stuff and this was the perfect opportunity. It is pretty easy to work with, creates a bond the rivals the strength of steel, and it's only kind of expensive (about $26 a tube in 2004). Also, welding was just plain out of the question. There was not enough room to maneuver the MIG torch into all of the tight spots, and this would have been an out-of-position welding nightmare even if I could. Here's the only downside to this adhesive, the applicator gun is a very unique and spendy piece of gear. Are you sitting down? It retails for $200. Fortunately, the cool folks at my local supply store (FinishMaster in Lakewood, CO) have a loaner gun.

I did lots of dry-fitting and practice clamping before I took the cap off of my expensive tube of goo. I had to use every clamp I had, plus a bunch of sheetmetal screws for the spots the clamps couldn't reach. It should also be mentioned that I thoroughly wire-brushed all of the remaining surface rust and gave everything a coat of POR-15 before sealing it up. Oh, and I used the adhesive to glue the fuel-vapor bracket back on too.

rear window 13

Instead of welding, the lower panel was glued in with 3M Panel Bonding Adhesive.

rear window 14

The adhesive was applied with this special gun.

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The lower panel was held in place overnight with clamps and sheetmetal screws while the adhesive dried.

Next Steps

I still need to remove the excess adhesive, and would like to fill some of the spot weld holes in the lower panel with bondo. Other than that, this repair is basically done.

rear window 16

The completed repair. It still needs some finishing touches, but most of the work is done.

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