SO i had my first time using a spot weld cutter yesterday. First one went great. THEN i was cutting and saw daylight after the cut. Nice little hole through both panels. Anyone have opinions on the best way to deal with this error on my part? As far as I understand I will need to make a plug and weld in a repair patch.
Thanks for any suggestions.
Yeah, stop before you get to the other panel 
seriously, it’s gonna happen. Usually you can just fill in the hole with the mig welder and do a little grinding.
If it is possible to reach behind the area you were drilling, use a flat copper bar flush behind the hole when filling it back up. If your welder is set correctly for sheet metal thickness, it will prevent the welder from blowing a bigger hole in the sheet metal.
If you can drive a small or thin chisel between the panels near the spot weld to apply some separation force, sometimes it will pop the panel off after you get through the first metal thickness.
I’m just about 3/4 of the way through my cowl project. I had the same problem with a few of the spot welds myself. But I’ve been a welder for about 30 years (construction worker) and like relentless said, try and get behind it with a bar. We always use brass bars not copper. But even so, if you can get your welder cool enough and take your time to let the bead cool every so often, you can fill the holes without a problem.
Thanks for the input guys. Ill either be trying the brass or copper backing. ill let you know what works.