I’ve got a 2003 X Type with some sill rusting. Not as bad as the other posts here that show complete sill replacement. We can’t help the guys whose cars already have serious rust but how do we save the rest of us? Why does this happen?

A clue is that my left side shows more rust. When you look at the panel below the windshield that covers the wiper assembly and cabin filter you’ll notice that the grill openings in that panel are on the left side as well. If you take that panel off (as if you’re changing the cabin filter) you’ll notice that there are openings at both ends that disappear down into the A post. I’m in the frozen north so I can’t do this now, but someone can test this by running a garden hose into the grill and let’s see where the water comes out. My guess is that the water (and salt) comes off the windshield, through the grill on the left side and down into the A post and exits through the sill drains. So what we’ve got to do is somehow divert that water. Let’s put our thinking caps on and see what we can come up with.

Having a whole bunch of water in the A post is probably why the auto transmission computers get wet and fail as well. More evidence.

 

I haven't really looked into why the sills of this car rust so bad, but I personally just wash my car the first day after a snow storm with fresh water to stop the salt from getting into the frame and swiss cheesing it.

 

Well…thinking caps must be in very short supply.

I did take the cover off and, sure enough, when I poured water on the windshield runoff goes down into the A pillars and then out the sill drains. Talk about a stupid, stupid design. I made some modifications to keep the runoff away from this area. Hopefully this will help.

Some observations. Washing the car probably doesn’t help much because all it does is wet the insides of the confined A posts and causes even more rusting. If you’ve replaced your sills, at great cost, you now have raw metal inside (particularly along the new welds) that gets wet and this is the perfect home for rust. A good rust proofing done by someone who really knows what they’re doing could help.