Backing tape
A heat-resistant tape with woven fiberglass center used to support the root pass and retain purging gas.
Stick the backing tape on the back side of the weld so the purge gas doesn't escape and no air reaches the root. It's made of fiberglass and withstands welding heat, so it doesn't burn away right away. Without taping, the scale on the root burns, the weld becomes porous, and on stainless steel you lose corrosion protection. The surface must be clean, dry, and free of grease, otherwise the tape won't seal. Stick the tape wrinkle-free over the gap and leave a small opening at the end for gas to escape. Before striking, purge the root briefly with backing gas until the air is out. Leave the tape on until the weld cools, then just peel it off.