Blotchiness can come from a number of reasons. Rick Crocket remarked on a number of them. You could also have a film from the solvents in the paint and the best stuff for that (if the normal method doesn`t touch the problem) would be xylene and/acetone - you can get some from the Home Depot (in the paint dept.) and clean like you would with anything else, making sure that you don`t scrub the surface. I would assume that the blotches kind of look like yellow and purple/blue sort of blotchiness and that sort of thing won`t really affect the quality of the mirror by very much as it`s a coating of the solvent and is just providing a interference fringes due to it`s thickness when looking at the finish.