I use Rustoleum Red Damp Proof Primer, others use the G.E. brand of Glyptol and Eastwoods also. Eastwoods doesn't sem as good as the G.E. brand the main thing to do before painting is to get the block super clean and free of oil,I use laquer thinner as the last step in cleaning and then use a heat gun, hair dryer or a propane torch to dry the complete valley any moisture to get it out of the cast iron. Hold the heat source on the metal(20 to 45 seconds or so) until you see the water boil up out of the cast iron, sometimes it takes two or more passes to get them completely dry do it once and come back to where you started at and see if it will dry up some more before paintning some people on this forum thank that the Glyptol is the only sealer to use, Mopar factory racers where successfully using the Rustoleum Damp Proof primer back in the 1960 or earlier