Cleaning and de-greasing is important. I use grease cutting detergents like Simple Green (concentrate from spray bottle) or Dawn (diluted) with brush to do the cleaning. Then a rattle can self etching primer that is intended to be top coated. The dark blue gray paint from the filling station can be sprayed or brushed onto the cast iron and it will look just fine. If the valve cover or oil pan it is best to spray it as brush marks will show. The primer will help the paint bond and last much longer. I have some that still looks good after 20+ years. I have used a piece of cardboard to prevent over spray. Taping with a couple of layers of newspaper or paper can also be used. If a little paint eludes the barrier a little paint thinner on a rag or paper towel can remove it. Adding catalyst to the paint helps it cure quicker and also longevity. It means you have to remover over spray quicker.