Tom, depending on the wood you might want to consider placing bitumen between the anvil and the wood.
Some trees, oak particular, contain aggressive acids. The bottom paint will very likely wear off and expose the metal to acid.
I had a hammer laying on a fresh oak over night, the next day it looked like it has been a month in the rain
A cold anvil will draw the heat out of the working peace, don´t use a sledge while its too cold and warm the anvil up with smaller work first