Sometimes you have to fight the battle...

I just wrapped up some rust repairs to my W124 sunroof opening and I can say that if you love your baby, catch the rust early and address it properly based on your skills, materials at hand and location as well as your desire to tear into it.

At a minimum, scrap, sand and remove as much rust as you can, treat with a rust converter or wash, encapsulate with POR or similar, and then reconstruct the area as best as you can with JB Weld.

JB Weld is very strong, easy to work with and its epoxy based properties makes it ideal for rust repair as it also adds a very hard encapsulation layer to the repair. I have used JB Weld for years in numerous capacities, for reconstruction of rust damaged areas such as battery trays etc. Should work well on your SL trunk floor. The key will be to mix it or work with it when it is thicker so it wont run on you. Also feel free to use screen, fiber mesh or whatnot to strengthen larger areas.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)