Iron is porous and the best way to remove oil/grease from it is to bake it(Best way)or heat it with a torch/heat gun if you're doing it at home.......You can take a block fresh from the machine shop and go over it with a torch/heat gun and it will sweat oil residue from the iron....which is what lifts the paint in the first place. We're not talking "Red Hot" here, just warm enough that if there's dirty iron it will smoke......about 10 minutes with a simple propane torch will tell the story. After that you have nice warm block ready for paint. Spray on cleaners work but they mostly clean the surface. Some shops have better equipment than others, but I always sweat the parts first to make sure....medium coat of paint to seal it off....assemble the engine...final coat of paint....Done.