Clean the block thoroughly. I like to start with a carb cleaning solvent in the blind bolt holes and follow with a scrub-down with an engine degreasing solution like Gunk. Bore brushes are helpful in the galleries and passages. Finally, a good cleaning with warm soapy water, and a clean rinse has the block spotless. Quickly wipe the bores and machined surfaces with a rust inhibitor, such as WD-40, and blow the block dry with compressed air.