FYI.. I replaced my water pump last week and I used some Acetone /Laquer Thinner to clean the mating surfaces thoroughly. Still some of the grout wouldn't come out so I used sand paper to get it out. I spread a thin bead of clear RTV Silicone sealant on the pump surface before bolting it back. I let it cure overnight and added coolant the next day. I have driven the car for over a week and so far no problems. Hope this helps

smk_texas
93 400e
86 BMW 325 e