Should Plywood Boats be Sealed Both Sides ?

[denverd0n]

Were I building the boat myself, from scratch, I might encapsulate the inside. The problem with trying to do that with a boat that is already built is, are you absolutely certain that you will be able to get to every last inch of the plywood? Nothing at all is "hidden" where you won't be able to get to it?

Because if any of the plywood is hidden, then you've got a perfect place for water to soak in, spread, and be trapped inside the remaining encapsulation.

Hence, for an already-built boat that was NOT encapsulated while building, I think I would leave it as is.

[Wotname]

Originally Posted by denverd0n

Were I building the boat myself, from scratch, I might encapsulate the inside. The problem with trying to do that with a boat that is already built is, are you absolutely certain that you will be able to get to every last inch of the plywood? Nothing at all is "hidden" where you won't be able to get to it?

Because if any of the plywood is hidden, then you've got a perfect place for water to soak in, spread, and be trapped inside the remaining encapsulation.

Hence, for an already-built boat that was NOT encapsulated while building, I think I would leave it as is.

I hear you but it didn't stop me from epoxy coating a 30+ year old plywood Van DeStat Dogger (31'). Took few years to remove all fittings, get her back to bare wood inside and out, repair what was needed, epoxy inside and out, repaint and then refit everything!

[Blue Stocking]

Originally Posted by doocruiser

hi there,
i recently purchased a 34 ft plywood catamaran that was constructed using stitch and staple/epoxy glue method.the whole exterior has been finished with fibreglass mesh and epoxy resin.the guy that built the boat has not sealed any of the plywood,stringers or any of the internal woodwork.he is adiment that one side of all the wood should be left unsealed so it can breath and if it does get wet so it can dry out. other people i have talked to with experience have said exactly the opposite and reccomend i seal the entire inside with a brushed/rolled layer of epoxy resin. there is a lot of water in the air on boats and leaving wood exposed does seem concerning.
any advice about this would be greatly appreciated.
thks

IMHO, after having managed a lumber and building materials division of a home center for 20 years, and having built a number of boats utilizing various types of plywood, is,--there are as many answers to this question as there are types and grades of plywood. Marine ply can be had in Fir, Pine, Luan, Meranti, Sapele, okoume, (sp) and probably more I havent listed.
Having sold thousands of sq. ft of "mahogany" type material, I can tell you to not trust the Indonesia and Malaysia products, which IMO have substandard cores, with numeruos voids.
The BS 1088 stamp does not always guarantee a good product either.
Brunzeel and Thames were generally the better "mahogany types.
My experience has been that the higher the ambient humidity when sealing, the higher the likelyhood of rot.

[Ex-Calif]

Personally I would go with Atoll's suggestion to encapsulate or at least paint to the waterline. No matter how dry a boat is there is porential for standing water in the hull for many reasons.

Or I might seal all interior joints with fiberglass tape and resin. For me protecting the end grain is key as this is the weak link IMO.

my interior is ply. It's 30 years old and the end grain is seriously under attack. I don't know how long you want the boat to float but I can estimate 30 years till your boat sinks based on one data point.

[minaret]

A lot of people here concerned about exposed end grain. On a properly built ply hull all joints will either land on a structural member for fastening, be scarfed, or both. Bulkheads should be tabbed both sides. There should be no exposed end grain anywhere on anything structural. Shouldn't be a concern. The boatbuilding school I attended advocated leaving the interior unsealed as well. I too have seen both methods fail equally catastrophically. Ply hulls are just a bad idea.