The eggshell should be strong enough to withstand gentle brushing of sandpaper across it. Just enough to wipe the dirt, not to rub away any shell. I folded the sandpaper so the grit area was small, and only brushed at the thickest of the dirt/poop encrustations.
I wasn't sure what you will be candling with. If not powerful, and the eggs are really dirty, then you may not be able to see the veins. But at 10 days you can tell an 'empty' egg from one that has someone in it. Don't throw any away, though, since the cool periods may have slowed their development. At 10 days, you may still have only 7 days worth of growth. I hope that makes sense.