My son is using a 4' x 4'piece of sheet metal to use as a chalkboard. He will paint it with the necessary chalkboard paint. We're looking at using either a 1/4" piece of Masonite or plywood for a backing. I'm wondering what would be the best mastic to use to adhere the sheet metal to the backing. We will clean the sheet metal with acetone to remove any oil any film. Any ideas?