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Old 03-13-2012, 12:04 AM   #1
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Default Rubberized Undercoating on door sills

The rear door sills are rusty and I was wondering if I got some Duplicolor Rubberized Undercoating and sprayed a few coats on there, would that be the way to go or do you have other ideas?
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Old 03-13-2012, 12:34 AM   #2
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Default Re: Rubberized Undercoating on door sills

Will not do the job. Use POR-15
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Old 03-13-2012, 01:17 AM   #3
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Default Re: Rubberized Undercoating on door sills

You'll just add a layer to seal more moisture in there. If anything, it could make it rust worse. Sadly, rusty doorsills is a common problem with s-series, and unfortunately, it's probably a lot worse than you think. What appears to be just a bit of surface rust turns out to be significant structural damage, once you start getting into it. It's possible that you just have surface rust, but it's more likely that the rust started between the two layers of the door sill and has finally worked its way through the top layer. Investigate it further. If it's as bad as it typically is, I would use a grinder (or other comparable device) to cut out the top layer of sheet metal that makes up the door sill. Inspect the bottom layer (you will probably need to keep removing more and more of the bottom layer to get to the edges of the rust damage. If the bottom layer isn't rusted too bad, clean off all the rust, and make a patch for the top layer out of some comparable sheet-metal. Paint the bottom of the patch and the exposed second layer, and try to get in in-between the layers as much as you can. After an hour or so for it to partially dry. Next, apply a very heavy coat of POR-15 to the exposed surface and the bottom of the patch, put the patch in place, and use a few rivets under the plastic trim to fasten it in place (while it's still wet). This should fill in the gap between the two layers, and glue them together (preventing further rust from starting between the layers).

Normally, you would want to weld something like this, instead of using rivets, however, welding will burn off any protective coating and it will start to rust very fast between the layers. Welding it will make it stronger at first, of course, but it won't take long before the rust weakens it (you won't see it for a while because it will be under the top layer, but suddenly, the whole door-sill will disappear overnight).

Once it's dry, paint the top of the patch, and any bare metal POR-15, per the instructions. Use bondo, as necessary, to fill the seams. Finally, top-coat it with a matching black paint. When I repaired this damage on mine, I got a spray can at a local parts store (Advance, I think) that matched the factory color perfectly (I was amazed at how well it matched). That was 5 years ago, and it's still virtually impossible to see the repair (if you know it's there and you look for it). Unsuspecting passengers would never notice, even if they're very observant.

Now, if you cut out the top layer and find that the bottom layer is damaged too, you need a more significant repair. This makes up and important structural part of the frame of the vehicle, and you can't just rivet a patch in unless there's at least one good intact layer. If it's bad, you'll need to cut out the whole door-sill and either make a replacement yourself, or replace it with one cut from a rust-free car (which can be hard to find). This will require welding in the new piece. Unfortunately, the welding will burn off any paint you apply and even the remaining factory paint, so even if you paint the top surface very well, you may be re-doing it in a few years.

When I started to repair mine, the only visible sign was a tiny bit of surface rust under the plastic trim. I cut out the rusted area to find lots of loose rust in-between. The top layer was rusted half-way through for several square inches. I kept cutting away more and more and ended up with this: I had to cut away even more before I made the patch. I had to remove the rubber door-seal and the painted panel below, to cut off the side layer, too (not yet done in the picture).
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Last edited by PlasticCarsRock; 03-13-2012 at 01:24 AM.
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