Slag Removal Mechanism

After welding, a hard slag layer forms on the weld. You have to remove it completely before the next pass or final inspection, otherwise you'll weld over the dirt and get inclusions in the weld. Use a chipping hammer or wire brush to knock off the crust until bare metal is clean. Especially with multi-pass welds, this is a must, otherwise the joint won't hold properly and can corrode later.