Thin metal welding tricks

I am welding patch panels in my inner fenders. The patch gets thin in the bottom section and while I'm not having trouble sticking it to the frame rail lip I am blowing small holes in the gap (only 2 so far). I've read here where putting copper behind the work helps, my question is is it a thin sheet, wrapped over a body dolly say? Does the weld ever go through and attach the copper to the back side of the weld? Also I read about placing a nail head in the hole and welding it and grinding off the back side. Is there any type of nail to get or avoid? Sorry for all the weird questions here, new welder. My welder is a 110 v mig, wire .023 mixed gas and I have it on the lowest heat.