I would not use 500 on a block for the whole area if it was me. You stand a greater chance of breaking through in the area around the run. If you use it with a very small block of wood or your finger so you just hit the run and not anything else the 500 should be okay. If you are just using a regular block I would start with 800 or so. I am just always paranoid about sanding through. 500 will remove material very quickly.