After disassembling a black iron pipe union to change a water heater, do you use any pipe dope or oil on the mating surfaces of the union or do you reassemble it dry.

Welcome to AMHD jocar,

There is no sealant needed on union. Attach dry.

Chuck

This is on the gas line? Make sure to bubble leak test. A union joint is "ground" to match. Just make sure the 2 faces are clean, without corrosion.

Assuming that this is a ground joint union and not a washer type. I have used pipe compound on the two surfaces.
Although it is not required. Good luck, Tom