It is always best to spot weld to a clean surface. This means that preferably all oils and oxides would not be present in the weld interface. Any that are present will lead to interference with and an alteration of the contact resistance. In some instances, the dirt may be expelled as flash. Sometimes it merely absorbs energy and slows or alters nugget formation. Clean is good. Silicon Carbide clean is not desirable. It is a nonconductor. If it gets imbedded in the surface, surface resistance goes up and the heat distribution will shift and maybe make it difficult to conduct current consistently into or through the part. Three surfaces in the diagram can be affected. Two contact resistances Rc and the interface resistance Rx.
One wants to remove oxides and nonconductors not imbed new ones in the surface and leave them behind like silicon carbide.
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References: RWMA - Resistance Welding Manual 4th Edition