The protective coating may be a layer of low density polyethylene of 0.5 to about 1.5 mils thickness applied by extrusion lamination, for example, over the ink layer on each side of the backing board, or may be a coating of a lacquer or varnish such as a nitrocellulose lacquer, a polyamide type varnish, a phenolic alkyd linseed varnish or similar coating of equivalent protective properties, which