Depends on the sail material. Most sails
for larger craft,have the logo or number sewn on and can be removed. Smaller sails, made from a Nylon or synthetic type of material, usually have the logo or number stenciled on with a long lasting paint
or ink, however most synthetic type materials dont retain the paint
or dyes for very long, unless it has been put on with some form of heat process, and can be removed with patience and an assortment of solvents.. DONT !! apply heat or any caustic substance to any sail material, you will do more damage than good. H the boat.