Paul Brand's Opinion About Rustproofing

By Paul Brand, Feb 8th, 2003 edition, Minneapolis Star Tribune 

Question:

I have a '98 Ford F-150 extended-cab pickup. I've noticed rust on the bottom of the doors and bottom of the tailgate. I went to the dealer and was told that if this had appeared within the first 36,000 miles, the dealer would have taken care of it. I was told there is a problem with the way the doors are made, something to do with the spot welding and water being trapped. I can sandblast the rust off and repaint, but is there something I can do to treat the metal on the sandblasted surface before I repaint? Also, is there something I can spray or pour down the inside of the door to slow down the rust on the inside?

Answer:

How many times have I recommended professional rustproofing to prolong the life of a vehicle operated in Minnesota? I rest my case.

The problem is not with Ford, or any carmaker for that matter. The problem is simply this: Ferrous metals would prefer to exist in an oxidized state. And eventually, they will. Rust is inevitable, and is expedited on our vehicles by the corrosive environment we drive in.

The construction technique for most automotive doors is to fold and weld the outer door skin around the edges of the door structure or frame. This creates a "U" shape at the bottom of the door skin, which tends to trap moisture and water ends up inside the door from the window channel. Typically, there are several drain slots or openings in the bottom of the door to help drain this moisture. These can become blocked or clogged, allowing water to accumulate inside the door and promote rust. It's a good idea to keep these slots open with a plastic straw or similar tool.

Spraying a non-hardening rustproofing material inside the door with a tool inserted through the drain slots coats all the metal surfaces that might come in contact with water, which can collect inside the doors.

What you're seeing on the bottom edge of your doors is corrosion from moisture, salt and road debris that worked its way under the paint from the inside surface of the metal door skin inside the door. Now that the rust has begun to "blossom," the only solution is to remove as much of the corrosion as possible mechanically -- with sandpaper, grinder, etc. -- and then treat the metal with a rust converter or rust-inhibiting primer, and repaint.

You'll find several kinds of rust converters at auto parts stores, including products from 3M and Permatex.  These products "convert" the surface of the lightly rusted metal into a hard, black paint able material, and with luck prevent the rust from "blooming" again. Unfortunately, since the corrosion started from inside the door, and you can only deal with the surface rust on the outside of the door skin seam, you'll probably end up doing this every year or so, because rust never sleeps! Once it gets a "hold" of the metal, it will come back eventually.