I don't understand. I've always heard to use pencils (never magic markers) for marking eggs to hatch.
But it's ok to use nail polish?
The solvent in Sharpie markers evaporates in under a second and shouldn't go thru an egg shell. But nail polish uses a slow solvent and takes several minutes to dry/evaporate, giving it ample time to breach the shell. I hope the Polish doesn't hurt the baby in the shell!
I'm new to the nail polish approach myself, I've only people using wax or in one case someone talked about boiling (unfertilized) eggs and using the sterilized, pliable membrane to cover a hole but I don't have personal experience with any of it.