hdo plywood was originally created for use in making highway signs where reflective material is attached to the surface of the sign. To make a more durable sign, it was decided that a more robust resin layer was needed in order to provide a better surface for attaching the reflective material. These materials are not as commonly available as softwood plywood. Consumer building supply centers normally will not carry them, but lumberyards that cater to professions probably will. mdo plywood costs about half the cost of Marine plywood, or about twice the cost of softwood plywood; HDO is slightly higher.HDO on the other hand up until recently, wasn’t a good material to use for boatbuilding. While HDO is even more indestructible than MDO, paint won’t stick to it and not much else will either. The panel construction process is the same, the paper is a heavy phenolic-coated paper but the product is generally used to build concrete forms and molds. The surface is made slick (I believe with a castor-oil based mold release) so that the forms and panels can be used over and over again.