Plug welding is an alternative technique to spot welding where overlapping metal plates are joined together by a series of small circular welds. This method commonly is used in automobile manufacturing and restoration, where spaces are too small to fit a spot welding machine's electrodes. Instead, a metal inert gas (MIG) welding torch is used to weld in a hole drilled into the top plate to melt the lower plate and fill the hole with filler metal to create a plug.
Drill holes every 1/4 to 1/2 inch along the width of the top metal plate that you are welding with a drill fitted with a 7.5 mm drill bit.
Place the top piece of sheet metal over top the base piece of metal and clamp it in place so it does not move while you are working.
Place the wire electrode from the MIG welding torch in the center of the first hole so it touches the lower metal plate.
Pull the torch trigger to start an arc weld for 4 to 6 seconds until the hole in the top sheet fills with liquid metal. This will fill the hole with a plug and join the two sheets together at that location.
Repeat step 4 until all the holes are welded.
Always wear protective gloves and a welder's mask when welding. Do not weld near combustible materials. Always weld in an area with good ventilation.