It's been raining, so now that I've removed the retarded Texan dealership emblem off the trunk of the Mercedes, it's time to patch the holes.
Bondo isn't really the ideal solution here—it's really best at filling dents and dings and sanded out rusty patches and the lot. Whenever you're actually filling a hole, you really should use sheeting or, at the very least, some lattice to support the Bondo.
But, the holes I'm filling are only 3/16" in diameter; they're borderline enough that I went ahead and filled them in with Bondo.
The key to any auto body work is proper preparation, so I started out by sanding down the holes, cleaning everything out, and then wiping the whole mess down with lots of rags. In the end, I had a grease-free bare metal surface with 80-grit scores so the putty will actually have something to adhere to.
Now, the Bondo container gives some really odd proportions for mixing the hardener with the compound ("1 1/4 inch strip for a golf-ball sized blob, or 3 inches for twice that") so I just ignored it all and did things the old-fashioned way: use the pink Bondo putty scrapers to gauge the proper amount of red hardener to add to grey base. The putty scraper you see on the piece of cardboard there is sold alongside the Bondo packages, and is the color that you're supposed to mix Bondo to.
The instructions also say to use a stiff and easily cleanable mixing surface. That's crap, just use cardboard.
Oh, and be sure to mix all of the grey base compound with hardener. Unmixed base is nasty gooey stuff and stays sticky and obnoxious well after you've cleaned all of the mixed pink putty off of your pink putty scrapers.
So, this is the first dab of putty put down on the metal. I typically use more Bondo than necessary, and blend the edges down into the surface really gradually. In this case, the bare metal was so small that the putty blobs cover them entirely.
It takes around 30 minutes for the putty to set up enough to be sanded. It's fairly critical that the bulk of the material be removed at this point, because when it all dries in another 24 hours, it'll be far too difficult to sand.
Here I'm most of the way finished with the left blob. I like to take most of the bulk off with a handheld rasp plane, which shaves the putty down like a cheese grater. Once it's been flattened to within a few 32nds of the panel, it can be worked down further with sandpaper.
At this point, I roughed the surface up again and threw another layer of Bondo on. It's usually a good idea to double up the coats if too much came off on the first sanding pass, which is fairly typical.
Now, contrary to typical instructions, I usually like to leave the final sanding until the putty has set up for at least a few hours. I find that the newly laid putty will sometimes flake up when I try to blend it too thinly. Plus, the hardened putty is a bit more forgiving of oversanding, which is a bit too easy when trying to get everything really nice and smooth.
Behold, the finished product!
In the end, the surface should be absolutely smooth to the touch—there should be no distinction between the textures of the putty and the paint. At that point, work it back to 800 grit or so and hit it with a few coats of primer before working the color back in and finish sanding it up.
As a footnote, do remember that Bondo fumes are fairly nasty and that all that planing and sanding often results in a fine mist of uncured Bondo particles which will happily attach themselves to nearby body panels. Be sure to clean the car up with a good wipe down before actually shutting up for the night, or else you'll have to polish the entire car to get all the grit off.
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