After 10" or so I usualy move down the panel and start again. By the time you get to the end , the place you started to weld has cooled down.

You just dont want to get it to hot cause if you do you will warp the area you are welding.

Also invest in some good vise grip pliers to hold the metal in place....I own stock in the company....I have about 45 various types of vice grips at work

Where you are going to weld the panels together make sure all the paint, rust, etc is ground off. Also you might want to get some weld through primer from Eastwood or 3M.
Later,

Doug