BYYB Forums

David Heineman, A fellow Pocket Cruiser builder who is having trouble accessing the forum, asked me to post the following question:

"I'm trying to find out if anyone using MDO (medium density overlay plywood) has
been able to paint directly  onto the paper impregnated surface, or if
they've had to scratch/sand it a bit first, or  if they have had to go as
far as to  sand down to the first layer of wood before painting it, or if
tthey had to sand it off,  and  later (for external boat surfaces)  if they
were able to fiberglass onto the paper impregnated surface or if they  had
to go clear down to the wood before fiberglassing?  If you could post it on
BYYB or find out through any of your other sources  you've developed,  it
could sure help."

I'll forward any suggestions/recommendations. Thanks! Paul

MDO was developed for sign painters, so you can paint directly on the paper faces. Of course you need to prime if the MDO wasn't pre-primed. The paper face takes paint quite well and I don't see a need to sand.

Thanks, Paul! I'll forward your information.

David asked me to clarify one point. Is it also possible to apply epoxy and fiberglass directly to the MDO? He's using MDO on the bottom, it will be glassed on the exterior.

Hi Paul,

Years agoo when we were in close contact with the American Plywood Association on every project, I asked about MDO and they told me, "Not for boats! It's paper!" Even if the resin grabs the paper, will the paper grab the plywood?
We've always stayed away from it.

The paper is and has always been applied with WBP glue. In fact MDO has always carried the APA (voluntary) product standard of 1-95, which is the same standard of all APA rated marine plywood panels.

Several manufactures have used custom MDO panels for planking in their production hulls. I personally own a 1961 lapstrake Chris Craft that is planked with MDO. Except for the garboards, they're the same planks hung when built in '61.

I'm not sure whom you were talking to at the APA, but clearly they didn't know their product grades or panel construction certifications. The paper facing, if glued face to face, will fail along the wood fibers in a peel strength test. Of course this assumes you've used an adhesive capable of defeating the tolerance of the wood's shear strength.

Until about 10 years ago, MDO was a very good product for planking and other boat building uses. Since this time, the veneer count has gone down, with 3/8" and 1/2" being only 3 plys and unacceptable for structural applications. Void count and width, defect count and size also have increased in this time frame, making MDO's use very limited. In lightly loaded applications like partitions, furniture, cabinetry, etc. you'll be fine with MDO. If used as bulkheads or planking, it should be doubled to gain the veneer count.

Thanks Pete and Paul. Personally, I am now totally confused, but it's not my problem, so I'll just pass the information on to Dave. For my part, I just do what Pete tells me and proudly use ACX

Paul