MDO Plywood

I'm in the process of revising my 1994 Generation III jon boat. Extending the front deck and adding a step between the deck and floor. Both the deck extension and step will have hinged tops for storage. I'm using MDO plywood for the top surfaces and for the floor. The MDO plywood I have has only one side covered with the fabric, the other side is wood. I plan to use a non-slip deck type sealer on the "up" side instead of carpet. Which side of the plywood should go up? Thanks in advance!

https://www.plumcreek.com/Portals/0/downloads/productInfo/MDO.pdf

Good question. For me, the top side is going to dry out quickly while the underside might be exposed to dampness/moisture for months at a time, so I'd put the MDO overlay side on the bottom. You can protect the non overlay top side with UV stain or your non-slip deck sealer. The MDO's glue is waterproof.

I'm with you - no carpet - it's too heavy.

Thanks for the info. If there is anyone else out there who has experience with MDO plywood, I'd appreciate any guidance you could share. I'll post some pics soon. :USA1:

I use the MDO and HDO in my line of work (concrete construction). Put the shiny side up on top to prevent water intrusion from above. I leave the bottom side of my decking unsealed so the plywood can breathe, if you seal the bottom the water moisture has no way to get out of the wood... it's trapped! I don't know how you store your boat when not in use, but I keep mine covered on a trailer with the bilge plug pulled. My boat is in good repair, does not leak, and the bilge is bone dry all the time. The only time the bottom of my decking will see water or moisture is if my boat sinks... and that ain't gonna happen.

The smooth shiney side on top will also give you a better looking deck after your finish is applied.

How can the plywood "breathe?" More likely it will stay damp if the moisture below deck is trapped between the hull below and that impervious MDO surface on the exterior/top. Seems to me that the water would have no where to go but INTO the plywood?

If, on the other hand, the MDO surface is on the bottom side, then the moisure below deck can't penetrate the wood.

Also, the best reason to use carpet is to reduce slipping and falling. It also covers up cheap plywood or bad carpentry. If you don't want to carpet, then staining a better grade of plywood, as opposed to painting, looks good yet retains foot grip, although not as non-slip as carpet.

Clearly, the deck, no matter which side the facing goes to, ought to be designed so that it is easily removed. That way, below deck moisture can evaporate completely.

xbacksideslider said:

How can the plywood "breathe?" More likely it will stay damp if the moisture below deck is trapped between the hull below and that impervious MDO surface on the exterior/top. Seems to me that the water would have no where to go but INTO the plywood?

If, on the other hand, the MDO surface is on the bottom side, then the moisure below deck can't penetrate the wood.

Clearly, the deck, no matter which side the facing goes to, ought to be designed so that it is easily removed. That way below deck moisture can evaporate completely.

Click to expand...

A properly maintained boat will have little or no moisture below deck, 99% of deck rot happens due to water intrusion above the deck. Being exposed to rain, snow, wet swimmers, and water taken on while uderway and so on.

If your bilge is constantly wet, you have problems that need to be addressed such as a leaky hull or exposure to the elements.

As I stated above, my bilge is and stays bone dry... and the bottom of my 15 yr. old plywood decking looks brand new and is bare wood. The top of my decking was another story when I did my last restoration. I didn't know to seal the top side of the decking the first time I replaced the carpet and my decking suffered water intrusion from above from getting caught in the rain and washing/cleaning the carpet. My decking is CCA plywood so it didn't rot, but it did check (many small cracks) from being exposed to moisture from above.

On my last resto I sealed the top of my decking with 4 coats of epoxy resin:

It's like I have a sheet of glass between the top of my decking and my deck finish (vinyl) now and I'm not even slightly worried about my plywood when I get caught in the rain or wash/clean my deck.

Many thanks for everyone's input. I finished fitting the edge pieces of the deck and step between the floor and casting deck with my son's help! Now to cut "wedges" to put under the floor and on top of the ribs so that the floor is relatively level. All of the exposed surfaces will be get two coats of "Restore". It is advertised as a "liquid armor resurfacer", is flexible and covers cracks up to 1/4". I've never used this before but believe it will be just what I'm looking for.........slip resistant and durable. I'm also going to put it on the existing aluminum front deck of the boat. I've spent several hours removing the old carpet adhesive and am going to put a heavy coat of bonding primer on the aluminum before the Restore. This stuff is available at Lowes/Home Depot for about $25 / gal.

Jigngrub -

My assumption is just what you mentioned - water from above, not leaks.

You would agree that that water, from fishing, swimming, spray, weather, and spills often finds its way below deck? And, that no matter how little of it gets there, unless below decks are de-watered and aired out, it tends to stay there, below deck, steaming and penetrating? Wood rot, mildew, and aluminum oxidation follow.

I am not persuaded that complete sealing plywood is the best practice. The problem is that there will always develop some kind of chink in the armor, some worn spot, some split, crack, or peel - by which water gets into the wood. That then means that whatever water gets in, due to what's left of the "complete" sealing will now have a hard time getting out. For example, the "sealed" but rotten stringers that can render a fiberglass hull worthless.

The veneers of plywood are separated by membranes of waterproof glue, the exterior surfaces are thin enough to dry out quickly. The edges are the most important to seal, and as a matter of maintenance to keep sealed, to prevent water intrusion down the thirsty end grain of the interior veneers.