Plug welding is similar to spot welding. This old Triumph TR6 is held together with about a million spot welds. To replace panels and make repairs many of those spot welds were ground away or released with a spot weld cutter.
All welding is done with a Firepower Mig welder from NAPA using .023 wire and an argon/co2 mixed gas (C-25).
Rather than spot weld the new panels in, I chose to plug weld them. In plug welding a hole is drilled or punched into the top panel and mig welded thru that hole to the lower panel. A punch was purchased from one of the tool catalogs for under $20. For best results the lower panel should be bare metal. I found that a blast from a sandblaster into each hole (Speedblaster) was perfect. Crank up the heat one notch and fill the hole with weld. You get a great bond but there is a bit of grinding to hide the weld.
6/22/01 - Will post some pictures soon.