(Click on Pictures to Enlarge)
This is the panel prior to initiating work. We are going to eliminate the gas door hole but this same method can be use to install any patch panel.
Metal is trimmed back to good metal with no obstructions then
a series of holes is punched (not drilled) every 1/2" along the edge where the new patch will butt up against theold metal. Make a paper or cardboard track of the hole of the hole in orde & nbsan accurate metal patch.
Cut 2" wide metal strips and insert them behind the old metal and weld them into place through the holes,have them protrude about an inch into the opening.
Trace your template onto your sheet metal and cut your patch.
Weld the patch into place by spacing the welds far apart at first, securing the panel then go back and spot weld in between each previous weld.
When the welds are about an inch apart weld the panel solid skipping from one side of the repair to the other in order to keep the repair as cool as possible to avoid warpage.
Grind off high metal from welding
The first coat of filler is a fiberglass paste that is waterproof. This will stop moisture that may get through your repair from being absorbed by the bodyfiller and blister the repair. The repair should be sealed in the back when their is access but in many cases that's not possible.
The filling work is completed with Bondo type filler then a polyester putty (shown) is added as a final coat.
Finally, with the body work done, a good primer filler is applied and guide coated and after sanding you're ready for paint.