The ash is related to contact with the air, so covering it will help, although it doesnt always work and most soapers get it occasionally. I just plane the top off my soaps, but try and be extra careful not to let it overheat when I do a textured top. I still get those tiny little bubbles to a certain extent. Most people will not notice or worry about them. Its after all a hand made product, and like baking a cake it will never be perfect, which is part of the charm.