Tape 400 - 600 sandpaper to a piece of metal that is dead flat.
Put ink on the housing sealing surface and gently sand it in a x pattern until the ink is gone to assure it's flat.
Use the Permatex gasket sealer specifically designated for water pumps and such.
Lap it up and you'll probably find it's not flat. I've found that you may have to do it a couple times, until it stops warping.
Granite surface plate (love mine), piece of granite countertop, flat glass or tile, anything like that can be used as long as it is dead flat.
Figure-8 pattern works well too.
