Chris and Mike Ashton write in "The Domestic Duck" page 137, "Dirtier eggs should be thoroughly cleaned straight away, but reject any eggs that are very dirty - they will most certainly become a health hazard in the incubator." For washing they recommend water that is slightly warmer than the egg to prevent the membrane from contracting and drawing bacteria into the egg. Don't rotate the egg in the same direction they were cleaned. Dry them quickly on a drying rack.
They also had the names of some safe bactericides, but we don't have the ones mentioned here in the states.