When I have plug welded in the past I often found the arc grabs the rim of the hole, which is the cleanest surface of the whole shebang coz you just drilled it, so very electrically attractive! This is a worse problem where the under piece being joined is the rusty old car! It can then be impossible to get it to pick up the under surface which is what it looks like has happened in your pics. Basically what everyone else is saying, just putting my 4d worth in - I've got a lot of plug welding to do in the next few weeks on the van so in the same boat...

I like clecko fasteners to do runs of plug welding. You drill a little hole, 2mm or so, through both panels. Then open the outer hole up to 6mm or 7mm. Spaced no more than 50mm apart but ideally 25mm or so apart you can use the clecko fastener to hold both panels tightly together. The further apart they are the more likely it will do as Mike suggests and the arc will just head to the rim of the hole and not properly arc through the bottom plate.

Are they the temporary fastners u use when riveting? I have used nuts & bolts to do that but it's very fiddly on a long run... Worth it tho!

That's them, worth buying a bag and the tool because they're dead handy.

http://www.skinpins.com/catalog/product ... -type-p-19