When welding galvanized and other coated materials a layer of oxides and brass builds up on the face of the electrode. During the first twenty five to fifty welds while this layer builds up and develops equilibrium, the welding results can be inconsistent. This period is called conditioning.
Once this conditioning period is complete the welding operation will run consistently and only be affected by the normal electrode mushrooming which will develop with time. Mushrooming can be corrected by dressing the electrode or increasing current to reflect the increasing electrode face surface area.
Remember not to dress the face itself only the periphery so that reconditioning is not necessary on the face.
Reference: RWMA - Resistance Welding Manual 4th Edition