I've got a nice Borden Ice Cream sign that is porcelain. It is a two sided sign, one side is great, but the other has bright green spray paint on it. What will be the best way to try to get the spray paint off without hurting the porcelain colors? Paint thinner? Remover? I'm not sure if the spray paint is water or oil based and it has been on the sign for >15 yrs.