And I quote: "MDO plywood is a form of overlaid plywood. What this means is that MDO plywood starts with a core of overlapping veneer, much like traditional veneer core plywood. But instead of having a surface veneer of finished woodgrain, MDO plywood features a surface layer of medium density fiber, much like MDF. The result is a sheet of plywood that can typically be used once for forming concrete.

HDO Plywood

HDO plywood is much like MDO plywood, except that the density of the fiber surface layer is much greater. This greater strength means that HDO plywood can usually be reused several times for forming. Thus the density of the surface layer is the distinguishing factor between MDO and HDO plywood."

It's also used for road signs, other uses. I've seen boat's hulls built entirely of it with success as well. I've used it for years in bulkheads and bunk flats as it is smooth, no splinters (except if you cut it and a thin one is raised on the cut edge..owee), takes paint very well, holds paint well. My experience with it is no voids for the most part either, any I've found have been absolutely minimal. This is the first time I've used it for deck and cabin sides but I have used it outside for years with no finish or edge sealing whatsoever with fading but no other issues for my underhouse scuttle which gets weather of whatever ilk.

Some 'glass mold makers use it for deck flats, etc in the "blank" from which to make the fiberglass mold as it is so smooth. I've used HDO a time or two but really don't feel the need. Reckon I'll see how it goes for this boat but I expect great success.