Stitch welding is the only way to do this without getting too much heat onto the panel and to keep from burning through.

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I hammered and ground the welds down flat.
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I applied a layer of Duraglas. Sanded most of that off and applied a layer of bondo.
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I sprayed some black "guide coat" on and began block sanding it off. I try to get it as smooth as possible before applying a final coat of primer.

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