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Sheathing 101

Are you interested to build a boat like the W17 but feel intimidated by the thought of glass sheathing large sheets of plywood? Perhaps your woodworking skills are well honed but working with fiberglass is new. If so, this short article is written for you.

Sheathing basics

The best way 'to learn fast', is to start on a fairly large sheet and for the W17, I'd suggest a good place would be to sheath the underside of the cockpit seat panels, as together they take nearly one full sheet.
First, you don't need more than 7'–6" in length (2285), so 6" can be cut off the end of one full 6mm ply sheet. Now cut that sheet down the center to give two parts nearly 24" (600) wide. (Eventually, the inner edges will be trimmed with a slight curve and the ends cut to suit the deck (see plans), but for pre-sheathing they can be left rectangular.)

Lay one plywood panel down on a firm, flat surface—an outside workbench or garden deck works fine or else lay the ply on some construction lumber to keep it flat and well supported. Remember that if the surface is curved and then you glass it, that you'll be fixing that bend more permanently!

Dust and wipe off the panel so it's clear of dust. Now unroll some 6oz bi-directional 'boatcloth' over the panel and cut off sufficient length—adding say 50 mm. Trim the side to cover the panel and then roll up the glass cloth around a cardboard tube.

Now mix up the epoxy. If using WEST System®, start by mixing up 4 squirts per pump, using the slow hardener (or more like 10 for the much smaller MAS pumps). Thoroughly mix for 2–3 minutes and then thin with about 5% acetone. This will permit better penetration as well as be easier to spread, but do not add more than 7% acetone (1/15th by volume, and this only for the first coat on bare wood). Make sure the basic epoxy is well mixed before adding the acetone though.

Now locate a plastic spreader about 5–6" wide with a clean, straight edge. (Run it across the bottom of a plane if it's rough—or use a fine sandpaper on a board.)

Pour the resin over the plywood in a zig‑zag pattern with a 5–10 mm wide bead of resin. Then immediately start spreading this resin with the spreader (often called a 'squeegee', as used for window cleaning). Sweep the surface with the spreader until all the ply is moist. Then go over it again like you are 'clearing an ice rink of snow' ;-)
Of course, that will only mean something to those in Northern climates—but it works like this. Start at one end with the spreader inclined at about 60° to the surface and angled 10–20° relative to the ends. In this way, when you scrape over the surface, the excess will run off one edge of the spreader only and after a few passes, any excess resin can be collected along just one edge of the ply and scrapped back into the pot. This method will quickly make an even spread of resin on any large sheet—no brush is needed and therefore, none to clean ;-)

Now, starting at one end, place the cloth on the plywood edge and slowly unroll it, keeping its edge just over the straight (outer) edge of the plywood. You can easily pull the cloth (by grabbing a bunch of fibers) in order to pull out any 'cloth blisters'. Then again using the spreader and starting in the middle, sweep out towards the ends to make sure the cloth is laying flat on the plywood. The resin will start to soak through and first go grey, and finally become completely transparent with enough resin. This image will show you what to expect.

Now mix up another '4 pumps' of epoxy and hardener and pour it in a small stream over any area that is white or still light grey. Once again, use the spreader to spread it out and give it a few minutes to soak though the glass fibers before deciding if more resin is needed or not. There's no rush with a slow hardener and the cloth needs time to soak up the resin.


Once all looks pretty clear, make a final pass to remove all excess. Start at one end with a slight angle to the spreader and now keep it a little more vertical, closer to 70–80° from the ply surface. There's a good chance that you'll find you now have a small excess of resin that you can return to the pot or use to re-moisten any lighter spots. The finished surface should be fairly matt with the weave showing, as a glossy surface means you have too much resin.

That's all there is to it and unless you are also adding epoxy to the edges, you can do it all without a brush or anything else that's difficult to clean. Try it—you'll be surprised how easy it is!

WARNING: Just a note, do NOT lift the panel until after it is cured, as if it's allowed to bend while wet, the glass will be displaced and once cured could be quite a mess, with areas of hard, lifted cloth.


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