Sandy has the right idea, but keep in mind that if you use the right wire, sheilding application and technique, you shouldnt have any pinholes at all. Just remember that when you weld (spot weld or tack in this case) cleanliness is key! your base metal needs to be completely free of any paint and oil. Make sure after its nice and clean, when you make your tacks, make sure that your tacks overlap, so that you get good penetration and a nice solid bead all the way around. work your tacks 180 degrees of each other, and let cool a bit after every 4 to 6 tacks. try it on some scrap 18 guage sheet metal, you should pick it up quickly. Nubby has the right idea on welders also. you dont have to have anything fancy for body work, and I like to use a solid core/gas mix, so the weld is clean and leaves no slag.