Is this by a living artist? If so, I would recommend asking about whether he/she wanted it stretched or whether it should hang loose. In any case, I don’t think a coating is appropriate. It probably wouldn’t be removeable,it would likely change the appearance of any of the cotton that isn’t painted already, and, besides, what would be the point of it! I know that it makes people feel better to coat things to protect them, but putting an unknown material on a work of art for an unspecified purpose makes no sense. Unfortunately, the best protection is a Plexiglas box.