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      01-13-2020, 11:22 AM   #1
LakeSurfer
Private First Class
United_States
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Drives: E93 M3, Nissan, Jeep, Mitsubis
Join Date: May 2019
Location: Florida

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Repair the M3 Undertrays DIY

Im a DIYer who enjoys the challenge of taking care of things myself.

A while ago i asked the forum about a way to fix the under trays on my M3 that where pretty much destroyed. The majority of responses that I received were that they cannot be fixed and to just toss them and purchase new ones. I have no idea why i listened, moreover, why did i listen to people who haven't tried and wouldn't try to figure this type of stuff out for themselves.

Enough complaining.

The undertrays are made up of some type of plastic/composite and a layer of felt type fabric on the outside. To repair cracks, holes, delaminations and so on requires the use of a plastic welder. I used the soldering iron type of "welder". I failed a few times using the plastic that came with the kit so i ended up shaving small chunks of the actual undertrays themselves and using it like a skin graft. If you can do this, it is the best practice because chances are that it will be very difficult to find the exact plastic that they used to initially build the part. manufactures ABS might have a different compositions from one to the next.

1. clean, sand with sandpaper the damaged area.
2. heat up the damaged area with a heat gun or the welder.
3. when the damaged area is almost melted, lay down the clean grafted pieces of plastic and use the welder to melt them into the damaged area until they are completely mixed in.
4. sand and clean up the area as you desire.

Remember to use a caul, clamps or whatever else you might need while working on the part as you want to work on it in its final shape because the heat can change its shape.

For my repairs i built them up with new plastic much larger and thicker then the factory and melted some medal screen into a few of the more damaged areas. And if the damage area was 1 inch I would repair about a 2 inch area.


Stop listening to the masses.

-Mark