You just have to smile when you see a project car advertised, "Rust Free." There's no such thing, unless the car has just finished a Pebble-Beach-quality restoration! Even on your modern daily drivers, it's not hard to find rust on exposed fasteners, frame rails, brake disks and the like.
But, we do expect that modern galvanized cars to have bodies without rust holes, at least for the first few years. However, it's not hard to find rust holes in a 1999 model, so one of the reasons I was drawn to my 1999 BMW Z3 Coupe, even with 196,500 miles, was its straight, accident-free body.
There was one spot, however, that did show a little surface rust - the bottom of the front wheelwells on both sides. That corner had been sort of sandblasted by the years and miles:
The rust also wrapped around the corner a bit, including the screw holding the panel on, and there was a separate small spot on an inner rail:
This spot is not structural at all - it's part of a trim piece that is still in the parts system, for about $250 per side. It's number 2 in the diagram below:
But, there's no reason to go buying a new part just for surface rust. I had been wanting to try a product called POR-15. It has a very good reputation among auto geeks as a rust repair product that is effective, long lasting, and nearly indestructable. Indeed, once you put it on it's hard to get off! No solvent can remove it, and it's so hard, it really takes some effort to sand it off.
For this tiny repair, I just ordered the POR-15 starter kit for 25 bucks. It includes a little can of POR-15, and two other products: a metal cleaning compound called Marine Clean, and another metal prep compound called Prep and Ready. The kit was rounded out by a pair of good, heavy rubber gloves, and a couple of cheap paintbrushes:
I started by sanding the affected areas with a sanding wheel in a Dremel tool. I didn't go all the way to shiny metal, because these metal prep solutions often need a little iron oxide to start the chemical reaction. I just wanted to knock off anything that was loose so it wouldn't flake off later.
The Marine Clean seemed to etch the metal a bit. The instructions just said to clean with it, and then wash with water and dry, which I did with compressed air. The Prep and Ready is a little more involved, because you need to keep it wet on the surface for 20 minutes. I did that, refreshing the compound as necessary, and I soaked the fasteners (which I had removed) for the entire 20 minutes. Again, you flush with water, and then dry. I used compressed air, waited 15 minutes, and blew it dry again. The instructions say it must be "bone dry" before applying the POR-15.
The POR-15 was like black paint, maybe a little thicker. I used a small disposable artist's brush, since the repair area was so small. You apply a coat, wait 2-6 hours for it to dry to a slightly tacky state, and then coat again. Two hours was just right in my shop at 60 degrees F.
One of the internet complaints about POR-15 is that once you open it, it's done - the unused product is ruined by contact with air. Indeed, the instructions suggest that you put plastic wrap over the unused portion, or better yet, package it in smaller containers. I didn't have any of that in my shop, and besides, I used about a teaspoon or less, so the container was still basically full. I just cleaned the lid and top of the can VERY carefully so it wouldn't stick, and pounded it tight. Even it it turns out to be ruined next time I open it, I was pleased to have it for this repair.
So, time will tell - it looks better, even in black:
I do have a stupidly expensive tube of touch-up paint from the BMW dealer, so I'm going to let this cure a week, and then touch up the part that shows.
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