The flanges were TIG welded in place, welds dressed, and then worked to remove the remnants of the old filler panel. The old spot welds were removed using a cut off wheel in a die grinder. When the upper panel gets thin, the heat turns it blue. When you see shiny metal again, you have found the second layer, so stop grinding!
http://i5.photobucket.com/albums/y16...n/IMG_2411.jpg
This video explains the process a bit better...
https://www.youtube.com/watch?v=nb49MgRPzvY
Next to clean all the rust that was hiding between the layers. A tight squeeze!
http://i5.photobucket.com/albums/y16...n/IMG_2412.jpg
The filler panel we fabricated was positioned, and drilled for some 1/8" cleco's. Then another test fit:
https://www.youtube.com/watch?v=evBQvi5dsnw
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