Since I struggle with welding in odd places I considered painting both sides of both components, clamping them together, drilling a hole and backing it with a thick copper plate and welding it as is. The penetration wouldn't be an issue if the arc is always struck on the flanges rather than the copper.

On butt welds which are hidden I stitch weld both sides and if the seam is to be finished to the eye I only work the weld down smooth on the side seen. It is very difficult to stitch weld without missing a spot or working the weld smooth to see the tiny holes. If I don't grind the weld I wire brush it with a drill, wipe it with wax and grease remover then 91% iso alcohol, and shoot a three or four light coats of well thinned 2K epoxy on it.

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84 300SD
85 380SE
83 528e
95 318ic