I wipe the sealing surfaces down with lacquer thinner and a clean rag. I then put a thin layer of silicone on my valve cover and attach the gasket (rubber or cork doesn't matter, personally like cork). Let the silicone cure overnight. Then I attach the cover to the motor dry-No Sealant! My covers do not leak for several years and you can take them off and on without replacing the gaskets many times. This process works on pan gaskets too.