After I degrease the block, I apply Ospho and let it do its thing for 24 hours. I then go over it with a Scotch Brite pad to remove the white powdery substance. It is a byproduct of the rust conversion process. If it doesn't all come off. I soak the Scotch Brite with Ospho which dissolves it, then wipe it down with acetone. Since the chemical process has been completed, the white stuff doesn't come back.
Next day I prime with Rustoleum rusty metal primer, then paint with 2 coats of Bill Hirsch dark gray engine paint.
I have done this on 2 engines and they still look like new after 4 years.
