Never

the surface of the gasket should be dry, the surface of the transmission should be dry.

wetting the gasket could make you think theres a leak and will be hard to tell if there is or not. I clean the surface of the pan in the parts washer and brakleen the transmission then blow it dry, wipe again with a clean rag and blow it clean again.

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99 Jetta Mk3
07 Jetta Mk5
89 300E W124
10 E 350 W212
08 C300 W204
Benz's aren't mine