all about plywood

It’s all about making plywood survive… The recent projects on Centaurea have been the main sliding hatch, hatch “garage”, and cockpit sole. i finally managed to get some decent-quality plywood for these bits, and work is coming along on them nicely. The plywood i bought is for the hatch top and “garage” top, the four cockpit locker tops/seats, and the cockpit sole piece. Ech of these three applications requirees a little different approach and treatment of the material. First off, a few notes on plywood…

When my boss made a run to town for supplies, i asked him to use his discretion and pick up two sheets of “whatever is decent, but not too expensive”, ideally 1/2″. What he came back with was two sheets of 15mm interior cabinet-grade cedar-faced A/B plywood. $60/sheet, instead of the $100/sheet they wanted for marine-grade, and with more plies.
i am replacing the original cockpoit sole piece for a few different reasons. The existing solid teak piece is coming apart; the wood is salvagable, but only just. The existing sole piece “hides” the scupper openings under its two forward corners, meaning that to access the scuppers you have to pull up the whole sole. And finally, i’m on a drive to remove all the exterior exposed wood from the boat.
The original/existing cockpit sole is a removeable piece, a feature which i want to retain. The construction is of several tongue-and-groove teak planks, the outside plaks tapered to form a blunt wedge shape rather than a plain rectangle. The sole piece sits on a raised lip formed in the fiberglass cockpit sole margin, which itself is shaped to form a gutter that runs around the perimeter of the sole. The gutter is about an inch wide, and the gap between the sole piece and the cockpit footwell walls is about 3/8″. This, in theory, leaves more than enough surface area for water to empty through the two (otherwise hidden) 2″ scuppers.
The new sole piece is cut-out above the scupper openings. Yes, that makes it easier to drop something “down the tubes”, but it also makes it way easier to inspect the openings, keep an eye on drainage rates, and clear blockages. The original plank sole started life as something approaching 1″ thick, but has since been worn down to closer to 5/8″ thick. Since the new plywood sole is starting life nearer to 1/2″ thick, it needed some extra beefiness.
i feel that non-marine-plywood is acceptable if it is of otherwise great quality, is not to be bent/curved/”tortured”, and is treated accordingly from the beginning. In this case, i start by cutting out the shape and routing the edges with a 3/8″ radius roundover bit. i then sand the whole to both “de-fuzz” the routed/cut edges, and open up the pores of the flat surfaces with 80-grit. Then, i set the wood outside to get nice and hot in the full tropical sun. Bck in the shade, i mix up a batch of West epoxy, thinned about 20% with acetone. With everything ready, i apply the thinned epoxy to the pre-heated wood, and just keep slathering it on (especially the edges) until it literally will not soak up any more.
If you work fast enough, you can go right on to the next step, but if the penetrating coats of epoxy start to “go off” too quickly, you might as well just wait until the next day, wash with acetone or ammonia, scuff with 120-grit, and start from there. The next step is to wrap all the edges with some light fiberglass cloth saturated in un-thinned epoxy. i use tape or strips cut from sheets; just wide enough to cover the edge and wrap around both sides flat without “springing away” from the faces. Let cure, wash with acetone/ammonia, scuff, re-wash, and proceed with the nest step… Here, i lay a layer of light cloth over what will be the underside of the plywwod, cutting darts in the corners and letting the cloth hang and wrap over the edges. Again, saturated in un-thinned epoxy. Cured, the excess cloth is trimmed, any roughness of the edges sanded, and the work moves to the top side. Here, both to build strength and thickness, i applied a layer of medium-weight (1 1/2oz) fiberglass matting, then another layer of light finish cloth, squeegeeing the two layers together onto the plywood. The matting is only on the surface, but the cloth, again, wraps around the edges.
Then it’s fairing compound, sanding, fairing, sanding, fairing, sanding, epoxy primer, sanding, etc., before paint and non-skid. Where the quick-releasze sole fasteners pass through the plywood/fiberglass, i actually take out a 1/2″ hole, fill with high-density West filler, and re-drill the smaller mounting hole through that. This way, all the plywood edge-grain is totally encapsulated. It seems like a lot of bother overall, but it’s really the only way to give that plywood a chance of surviving!
For the hatch top/garage top, the plywood needs a slight camber put into it. The original top(s) were of splined teak planks, about 5/8″ thick (but now worn away to almost nothing). For the hatch, the underside of the plywood has gotten a series of kerfs sawn out of it. i put them in about every 1 1/2″, cutting 3/4 of the way through the plywood. Lots of kerfs makes for stress-free bending, and ultimately greater strength… The piece is slathered with thickened epoxy and clamped in place overnight. After curing, washing, and sanding, a layer of cloth goes over the whole top. To facilitate this, the edges of the plywood top are well-radiused, and of course, the same “hot wood/thinned epoxy” technique is applied to the plywood. The cloth is there for a bit of structure, but mainly to help seal away the fragile edge-grain. Now attention is turned to the underside, with all those open kerfs… first, another round of hot wood/thin epoxy to really soak into the grain, then the kerfs are packed with moderately-thick high-density epoxy. The cured epoxy forms tough “splines” withing the plywood structure, running 90 degrees to the curve of the camber; this makes for a very tough and rigid structure. The structure of the “garage” top will be substantially the same, but the kerfs taken from the outside face; as installed in place, the inner face will be inaccessable.
The cockpit locker lids/seats require another technique. The originals are more tongue-and-groove planks, with splines running full-width through the ends; not bad construction when new, but now failing. The same basic principles are applied to the plywood replacements here; “hot wood/thin epoxy”, layered edge-wrapping in glass, etc. These pieces are subject to a little more abuse though; the outboard edges have piano-hinges screwed into them, and the inboard edges are slightly overhanging into the cockpit. For this reason, i’m going ahead and edging all the plywood pieces with substantial solid-wood bullnosing under the ‘glass.
Again, a whole lot of bother… but when complete, under well-applied two-part paint, these plywood-and-glass pieces should outlast their solid-wood precursers, while looking good and making for less maintenance. i have no problems using plywood this way, even non-marine-grade; the trick is too be very aware of the limitations, and do everything you can to preserve the plywood from the beginning, for once a plywood core begins to fail, it is essentially a case of replacing rather than repairing it.


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