So Why MDO?
The hull of the True Grit was planked with Marine Douglas Fir plywood but as I moved to the remaining structure I decided to go with Medium Density Overlay or as it is better known - MDO. For those not familiar with MDO it is an exterior plywood sheet, which has a resin impregnated fiber coating that has been fused to the surface of the panel. The coating serves much the same purpose as fiberglass covering. Although not as abrasion resistant it does eliminate surface checking. Also, it's smooth surface takes paint well with little or no sanding. It is an engineered exterior rated plywood mainly used for outdoor signs, concrete forms and similar exposures. It is available with the coating on one or both sides (more on this later) and in similar dimensions as normal exterior ply. This is not an item usually found at the big box lumberyards like Home Depot but most "real" lumberyards either have it or can order it for you.
But why MDO rather than Marine? The first consideration for me was both cost and availability. As to cost it is about 50% the price of marine fir. Plus even at $110 a sheet for marine 1/2" I was not thrilled with the quality. It had more small voids that I thought it should and far more "football" plugs than I like. I have two lumberyards nearby that stock MDO and that allows me to inspect each sheet as it comes off the skid as apposed to a special order where you get what they get. Another reason was with the cabin and berth structure the interior face of the plywood panels forms the interior wall or ceiling. I wanted just to just paint them but if you have ever tried to paint plywood and hide the grain you are familiar with the difficulty. It can be done but takes a lot of work. Using two sided MDO solves this problem as it offers a smooth surface on both sides and require little prep for paint. I guess I could have used single sided (less expensive) and placed the finished face to the inside and then added the normal fiberglass cover. But my thought was why mess with the fiberglass if the resin coating will serve the same purpose. So may conclusion was it would be both easier and cheaper to use two-sided MDO and ultimately end up with the same result. So the idea seemed to make sense.
A little research on the web revealed that many builders are using MDO for boat building. Some are using it for both the hull and the superstructure. Many just paint it and are content with the result. I was intrigued so I decided to buy a sheet and experiment. One of my concerns was filling screw countersinks. The resin coating has the appearance of being a brown kraft paper type material. I was afraid when sanding the wood filler smooth the "paper" would fuzz and then show up on the painted surface. This I found to be untrue. Even sanding with 100 grit sandpaper and an orbital sander I can paint it with no visible defects.
It proved to take paint very well. On the interior surfaces I am using a gloss latex porch and deck paint and the first coat almost disappears as it soaks in. About five minutes later I can go back and hit it with another coat and it starts to build but usually four are five coats are needed to finish it off well. Using a fine foam roller and carefully applying the paint I am getting a nice slight stipple coating that is very smooth and has NO plywood look whatsoever.
On the outside I decided to take and extra step and coat the exterior with epoxy. Although paint would probably been just fine I liked the idea of really sealing everything with resin. I found like the paint the first coat of epoxy soaked in almost completely but after that built up well. It will be sanded and prepared for painting as usual ( probably EasyPoxy again).
Because of my experimentation I decided MDO was a workable solution for me. But it is not perfect as there are some downsides. First, the exposed plywood edges still need to be dealt with. Some builders handle it by just adding a solid wood banding to cover the edges. This might be an easy answer in some situations but I personally wanted a "cleaner" more modern look for my True Grit than this would provide. I decided to radius and then glass the edges at the hull side / deck intersections and similar locations. These were just feathered into the flat surfaces as normal. Then there were a couple of places on the cabin walls were sheet edges were butted together. On the exterior I routed a shallow groove about 1 1/4" wide and inlayed three layers of 1" glass tape. These were sanded flush to the surface. On the interior I used a butt block to backup the joint. I plan to use the same method on the cabin roof with the joints landing on the top beams, which will eliminate the butt block.
Another item worth noting is MDO is not as hard as Douglas Fir. This is nice as countersinking screws is easier but I would think it is more prone to impact damage such as dropping something on the surface. Another concern, depending on how it is being used is the integrity of glue joints. Because the coating appears to be somewhat "paper" like, I decided to make some test glue joints where I attached solid wood directly to the ply. Since I was using PL Premium Construction Adhesive I wanted to know how it worked with the MDO. Previous testing with DF ply proved a strong bond with wood fibers tearing away when the joint was destroyed. But the MDO / PL Premium joint did not fair as well. I tried non-sanded and sanded surfaces of the MDO but all failed at the resin coating line. The coating always separated with no actual wood fiber being pulled. In my book this is a weak joint and I would avoid it in areas of stress. NOTE - I did not try the same test with epoxy but my guess is that the joint would be much stronger as the thinner epoxy would probably penetrate the coating better. If this is true it would make the next two paragraphs a moot point.
The weaker joints would appear to limit MDO usefulness with PL but no, there is a fix. It takes a little extra time but I decided to just remove the resin coating where I needed the strong joint. My normal building procedure is to position the ply sheet in place and outline all structure where screws are to be used. I then remove the sheet and drill the holes in the ply centering the screws on the outlined member. After that I used a router with a bit set just deep enough to cut away the coating revealing bare wood. Using a clamp-on straight edge I could do this accurately and fairly quick. On curved outlines I had to go a little slower and free hand it but still not a big deal. Below is a photo of half of the berth top with the coating stripped away. The area to the left of the curved line will be trimmed off but I find it easier to cut away the coating if both sides of the router base can be supported.
I do not do this all the time. In fact, most of the time I don't because all of the joints are backed with screws. I don't see the need. For example, the storage boxes in the cockpit (previous page) were all built with MDO screwed to wood cleats at the corners. Here and places like it, I feel the removal not worth the effort as the strength is just not needed. Also, when I install the 3/8" MDO cabin top I do not plan to remove it. Since the beam spacing is 12" I feel there will be ample strength with all the screws, even with a less than ideal glue joint.
Another negative comment I have read about MDO is because of the varity of wood species used it has an unknown rot resistance. We know Douglas fir receives a fairly positive rating in this respect but MDO with mystery species of interior plys leaves a question mark. As a result, some builders shun its use. I find it interesting, as often the same builders will sing the praises of Okoume and Meranti. Not sure if they have looked but my findings are both of these have very poor rot resistance. To me the point is ANY PLYWOOD needs to be sealed from moisture or it will all eventually rot - some faster than others but still rot.
My final comment is all MDO is not equal. Just as with Douglas Fir Marine Plywood some is of better quality than others. I have found that double sided at least appears to normally offer better surface quality - at least on the best side. Usually one side is better than the other. The poorer side may have more grain projecting through the resin coating. My one supplier has single sided 3/8", which I judged of too poor quality to use. Not that there was knots or voids but the surface was just not smooth and would show when painted. I found this the hard way and one piece did get installed in my refrigerator cabinet. Every time I walk by it I sigh but ripping it out would be far more troublesome than it is worth. Most of it will be hidden by the chair sitting in front of it but the upper section is in plain view. Maybe a picture or wall hanging of some type will hide the defect. We'll see. If you have a sheet that is a little textured try sanding it with an orbital sander and 100 grit sandpaper. Most of the imperfections can be minimized.
So that is my take on MDO. Personally, I find it very useful in boat building and would use it again. But am still not sure if I would use it for a hull of a larger craft or one that was going to sit in the water a big part of its life. It may work just fine but I would have to give it a lot more thought. But for the superstructure if properly installed I feel it is a good choice. Just my 2 cents.