Finishing the body patch

We're finishing the body patch we started last weekend.  Continue to tack weld the seam of the patch.  You cannot do continuous welds! Too much heat will warp the metal.  The only way to do this is to do a million tack welds until the whole thing is done. Here I have started the tack welding.

Here it is almost all done.

Grind the welds down to get a look at the progress.  Now we can find all the spots we missed.  Go back and do those.  Regrind. What you want to get to is a continuous flat metal surface.  Go over the whole area with some 80 grit sandpaper to give the metal some tooth.  Clean the whole thing off with some wax-and-grease remover.

We're going to use body filler to smooth out our panel. Mix up some filler according to directions and lay it over the whole area with a plastic spreader. Don't be too fussy, most of it will come back off anyway. You should have something like this.

I just love love love the smell of body filler. Now we're going to knock the filler down with a "cheese grater" file. I use just the blade part.

You have to wait about 10-15 minutes for the filler to get semi-hard.  Don't start filing too soon, you'll just clog the grater and pull too much filler out.  Start filing, and the filler will come off in strings, like this. I like to flex the blade a bit with two hands, and I usually work in diagonal strokes in both directions, sort of a cross-hatch pattern. Work on the filler, flattening it until it is close to the contour of the panel.

When you're done with the grating, it should look something like this. I have removed probably 1/3 of the filler doing this step.

Now I use some 80-grit sandpaper on a simple rubber sanding block. More long, diagonal strokes. Get the filler nice and flat, and feather the edges out nice and thin. You can see bare metal showing, these are the high areas.  There's also some unsanded areas in the middle of the filler, these are obviously the low areas. Apply more filler in the low areas, and resand.  Eventually you will have a dead flat panel, with the filler even and blended into the surrounding areas. Sanding should remove at least another 1/3 of material.

Add filler and sand as many times as needed. Use your hand to check the panel.  Just looking at it is good, but you have to feel the panel to find any uneven areas. Yes, there's a bit of an art to this, but it is developed from experience. You should end up with a thin coat of filler that basically brings the low areas level with the high areas.  Filler should not exceed 1/16" thick; any more and your metal work is just too low. Now sand again with 200 grit.  Nice and flat, isn't it?  Feels nice, doesn't it? Spray the area with auto body primer, the spray can kind is fine for now.  We will go over this whole area again with 400-grit before paint.  Here is where you should be (yes, this is the other side of the car, done previously).

So that is the crash-course time-compressed version of panel repair. I have a ton more tips and tricks I don't have room for here. I have to warn you, this is a lot of work. Just this rocker repair and panel patch on one side took me about 10 hours. That's assuming you have the tools to do it properly. I like doing this, it's part science, and part art, and it's nice to have a sturdy repair that will last a long time, where before we literally had nothing.

Other stuff:  I will spray the inside of the repair with paint, and a couple of coats of undercoating.  I can do this from the inside since I still have to repair the inner rocker panel. Sealing the backside is important to the longevity of the repair. Wear a mask when sanding filler, it's probably bad for your lungs. Don't expose bare primer to moisture too much, it is porous and moisture will go right through it.  Body filler contains a lot of the mineral talc, and if this soaks in too much moisture, it will expand and ruin your nice flat panels.  So keep this dry as much as possible.

There it is, how to fix a rusty panel.  It's not rocket science, but it does take a little skill and a bunch of patience; both of which you can learn. And no matter how much you mess up, it's fixable.  At the worst, you will have to cut it all out and start again, and you will have more experience than before.