Discard eggs that are damaged, cracked, unusually shaped, have thin shells, are too large, too small or soiled.
Eggs should not be washed because this removes a protective layer that coats the eggs. Washing eggs can contaminate the eggs as bacteria and disease from the surface can be transferred through the shell to the inside of the eggs.
Eggs should not be wiped with a damp cloth as this also removes the protective coating and can therefore allow the transfer of disease and infection through the shell.
If soiled eggs must be used, then these eggs could be incubated separately from clean eggs, to prevent cross contamination.
If soiled eggs are to be used, then they could be cleaned using a fine sandpaper that gently rubs the soil and debris from the shell without transferring it through the eggshell. Be careful not to rub too hard as this will remove the protective outer layer of the shell.