Remove all oxides on the surface of the sheet with a metal brush. Remove oil and dirt with a degreaser.
Fire the welding torch and direct it at the metal sheet, to create a welding puddle. Direct the arc at the middle of the welding puddle; keeping the arc at the edge of the puddle enables the puddle to insulate the base metal from the full force of the arc. Do not whip or weave the torch. The longer you keep the torch in the same area, the hotter the metal will get. Move as quickly as possible across the metal sheet, while maintaining a good bead profile. Be sure not to leave gaps between the sheet and the part or wire to which you are welding the sheet.
Use a skip welding technique to distribute heat as evenly as possible. For instance, when welding stainless steel to repair the side of a tank, make a one-inch weld, then skip six inches, then make another one-inch weld, and continue the process for the length of the stainless steel. After you've traveled around once, make your next one-inch weld three inches from the first one-inch weld. Continue to place each of the second set of welds three inches from the original set, and continue the process until you obtain the integrity that you desire.
Clamp a backing bar, usually made of copper or aluminum, since they dissipate heat best, to the back of the weldment. In higher duty-cycle applications, consider using a water-cooled chilling bar.
Avoid over-welding. Excessively wide welds waste time, filler metal, and gas, and may affect the temper of the metal. When welding a 1/16-inch plate to a 1/8-inch plate, the weld need only be 1/16-inch wide.