In this application they are successfully spot welding three thicknesses of 0.019 SS together. When they switch the outer two layers to aluminized coated steel issues arise. They experience need for tip dressing and weld setting changes.
Stainless steel is a relatively resistive material as compared to plain carbon steel and certainly aluminum. When the two outside materials in the assembly are changed to less resistive materials (low carbon aluminized steel), the heat from the part resistance will be lower. More time or current likely will be needed to maintain the same nugget growth.
The electrode for spot welding stainless is either RWMA Class 3 or 2. If the material is changed to aluminized steel RWMA Class 2 would probably be preferred since a higher current may be used to make up for the lower total resistance in the total relative resistance.
As to the coating pickup and added dressing due to aluminum pickup? It will happen. Aluminum melts at a low temperature and is going to be picked up by the electrode face. Welds can be made through it but regular dressing needs to be performed to maintain consistent weld quality.
References:
AWS Standard C1.1 Recommended Practices for Resistance Welding
RWMA - Resistance Welding Manual 4th Edition