All spot welding electrodes mushroom during use and when this happens the resultant weld eventually is unacceptable. Before this happens most operations either dress or change the electrode to get the face back to original operating conditions.
The worn electrode should be dressed on the machine or in a secondary site by mechanically machining the face back to nearly original dimensions. On the machine this can be done with hand held pneumatic dressers with special contour blades or ratchet style hand held dressers with contour blades. In robot/weld gun installations the robot can move to a dressing station and the electrodes can be dressed at periodic intervals. Electrodes removed from the welder for refurbishment can be machined with similar dressers or on a lathe.
In all cases the weld face is normally not machined if welding galvanized material in order to avoid creating the need to recondition the face of the electrode. Conditioning means the first twenty to fifty welds on galvanized, when the electrode is developing a consistent layer of oxides and brass on the face. During this time welds are generally very inconsistent.
Reference: RWMA - Resistance Welding Manual 4th Edition