bab are you talking about the combustion chambers in the heads?Or the cylinders themselves? You said new heads so I guess you're addressing the block. Light rust will be removed by the rings very quickly as you cycle the motor over. Heavier deposits will require some 240-320 grit lightly scuffed to remove the rust. Either way some light oil spray will help, and any residue should be blown out or wiped down as you go. If you're saying HEAVY rust, stop. Clean enough to get the pistons out without breaking them and take it to a shop for some honing....... A SOFT type rotary wire brush on a drill will clean the tops of the pistons.......................Moneypitt