When used in building structures, the Welder will first take any available Components required from the player's inventory and place them into the incomplete block. Further use of a Welder on the block will slowly increase the construction progress as long as the required materials are present.
Damaged layers can be repaired by replacing components in those layers (e.g. if a turret is damaged and its integrity decreases to 80%, you most likely have to only replace the first few layers of steel plate). Dented blocks may not require any resources at all to repair; simply using the Welder on such blocks will slowly repair them.
Using the Welder consumes Suit Energy, but only when actively building or repairing a block. The rate at which the Welder makes progress is influenced by the "Welding Speed" World Setting. In multiplayer, the rate of welding progress stacks additively for each player welding the same block. (that is, three players welding the same block will be three times as fast as just one player)
The player always spawns with one Welder in their inventory if enabled in World Settings, as well as a Hand Drill, Grinder, and an Automatic Rifle. Additional Welders can be constructed in an Assembler.