Posted by Michael Hanson on September 10, 2002 at 18:07:19:
In Reply to: marine ply posted by carsen banister on September 10, 2002 at 16:19:54:
:In general, there are three grades of plywood: inerior, exterior and marine (or areo-marine). The plys of interior plywood are glued together with resin that dissolves in water which causes the plys to delaminate(quickly). Obviously, it is unsuitable for building boats. In contrast, the plys of exterior plywood are glued together with resin is resistant to water, so plys take much longer to delaminate. But, commercial grade exterior plywood has four other disadvantages when compared to marine grade plywood. It is usually made of low quality undried pine, so it weighs more, weathers poorly, and has a raised grain that increases drag. It also contains hidden "voids" in the inner plys (places where there is no wood at all, sometimes all the way through to the other side, and sometimes only knot-hole sized) where water can accumulate (leading to rot) and which creates weak spots in the hull (particularly in areas where the plywood is bent). It also usually has thicker, and therefore, fewer plys, which makes it structurally weaker and prone to extreme warping (it is the number of plys that determines the strength of the plywood and its resistance warp ). Finally, pine can only be made to look so good with stains and varnish. Even covering it with paint is futile because its characteristic grain usually shows through after several coats of paint (because it is raised). Marine (or areo-marine) plywood, has none of the disadvantages of the other types plywood discussed above, so its advantages are obvious. You can't get it from Home Depot, but it is worth the price and the time spent waiting for shipment. Oh, it is also absolutely beatutiful.