Door anti-rust preventative maintenance

When I was viewing 328s before I bought one, at least two of them had tiny rust bubbles along the bottom of the doors. This is one of the points to look out for.

Having taken off a door trim today, its easy to see why this happens! The design could not be worse for trapping water. Inside the bottom of the door, there is a V-shaped channel which is formed by the inside curved face of the door skin and the vertical flange of the door bottom. This is below the level of the drain holes in the door bottom because the flange turns downwards. So any water running down the window into the door will go into this channel and just sit there, doing its evil work. It will also tend to percolate down the joint (at the bottom of the V) where the door skin meets the flange, probably right down to the bottom inside of the skin.

On my own car it looks like the factory sprayed the inside of the door with brown Dinitrol, the stuff they used for rustproofing pretty much everywhere, but they didnt angle the gun down into this joint along the bottom, so there was no rustproofing at all just where the water gets trapped. In fact surface rust on the inside was already visible and so eventually it would be a matter of time before going through.

So.... what to do? After some deliberation I did this:

Brushed Kurust along the inside of the V shaped channel. This is very liquid so will hopefully run down in between the door skin and flange of the door bottom.

Allowed to dry then brushed Dinitrol all over the inside bottom of the door, working well into the channel and then working upwards on the inside of the door skin for a couple of inches.

Filled the V shaped channel with Silicone sealant, so the result is the entire inside of the bottom of the door is pretty much flat. Any water goes through the drain holes instead of sitting in the channel in contact with the inside of the door skin.

If anyone is interested I could try to get some pictures when I do the other door.

 

Here is a rough sketch showing a cross-section of the door, and where the problem lies. The solution is to fill that V shaped area with something waterproof, so there is no void below the level of the drain holes.
Image Unavailable, Please Login

 

Thats a fair point, but if water gets in between the outer skin and the door bottom at the point where they are flat against each other, it cant evaporate already. If filled properly water wont get in. Using silicone I was able to get a radiused curve in that corner which directs all water towards the drain holes.

 

Thats fair enough but never wash the car?