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Start by marking off the section to be repaired with a straight edge.
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Use a die grinder to trim the bottom edge of the skin and around the corner. Remove just enough metal to allow the skin to separate from the frame. Note all the corrosion that builds up from water and road salt collecting in the crevices.
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Use a cutting wheel at the edge to make a slot for the air saw. An air saw is ideal for doing straight cuts on flat panels. Use the cutting wheel on curved surfaces and corners. The air saw works fine in the straight sections.
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After removing the piece you've cut away, examine the doorframe for additional damage.
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Measure the piece you've sliced off the door, and allow some extra material for "fudging." You have to make the piece big, and Prosser recommends fitting it to the car, not the part (which may not fit exactly on the car in the first place).
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A metal shear is ideal for cutting, but for small parts a pair of tin snips will work too.
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Use a pair of dividers to transfer the dimensions from the original piece to the repair patch. You'll be working in three dimensions, so you'll have to take several measurements to get the right shape. Simply score the metal with the tips of the dividers.
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For curves, you'll need to scribe the piece against the repair patch. After scribing with a pencil and laying down masking tape, brush on some Dychem to stain the part, so it's easier to cut and shape it.
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Again, transfer the shape of the fold to the patch with your dividers, and use it to make guide marks on the stain.
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Check your cut against the original part to make sure it follows the contours correctly. Use a file to remove any burrs.
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Now your patch is ready for shaping in a flange or edge turner. The wheels and attachments for this tool allow you to make a wide variety of shapes and sharp curves.
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The shrinker/stretcher allows you to put long bends in the piece. Use a hammer and metal backing block for some gentle persuasion.
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A professional-grade press brake runs about $1000. You can get a small one for about $150, or even create a cheap substitute with a piece wood against a some-angle iron clamped to a table.
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Here are the curves the press brake imparts to the patch. You may need to fine-tune the bends with a hammer, to make sure it matches the original part.
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Now you're ready to weld the patch on to the doorframe. Note the excess at the ends, which will be trimmed back.
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Use an air saw to cut a slot and bend the ends around the frame. Afterwards, you can then weld on a new flat panel, and finish the seams.
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