When using only water instead of cooling fluid aluminum-oxidation can occur. This seal face on the water pump is extremely eroded at a few locations.
After welding, the seal face will be mechanically machined.
At the last step protruding burs and redundant aluminum will be removed.
After that the water pump is watertight again with the original seal without using sealing mass.
After that the water pump is watertight again with the original seal without using sealing mass.