Ace industrial paint works very well. It's cheap and locally available. Clean the casting with Simple Green and a ScotchBrite pad to remove any and all grease and oil. Scrub it with another piece of ScotchBrite with hot water and Dawn to remove the Simple Green, then wipe it down with a wax and grease remover. Don't touch it with your oily hands during this last step. Mask it if needed and paint. Brush will work if you thin the paint about 10%.
Mark