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Sheathing Part 1 — Sheathing Flat Panels

To sheath a large panel, like a boat side or deck panel, you need a large flat surface to lay the panel on. This is why, in the W17 Build Manual, I suggest to use the building platform before you actually set up for any hull assembly.

Pass a sander over the whole surface so that you can check if there is any imperfection that first needs to be either filled or sanded down. Then brush or vacuum off the surface and make sure the platform around the part is also dust free.





Select the roll of cloth that you will be using and after considering all the other panel widths to be sheathed, see if it makes sense to cut the roll down for convenience.

For the W17, I figured that I should cut a 50" roll into two, with one at 19" and the other at 31". While the supplier told me to unroll it (20+ yards?) and use sharp scissors, I have found that if I tape it up tight near the planned cut, that a sharp 'hard-tooth' back saw will quickly saw through the glass down to the tube, without wasting more than about 1" (25 mm) total.

Then pass a rope through the tube and suspend the roll you need over the work table, above the panel to be sheathed. Pull out a length of cloth to cover the part, with about 2" (50 mm) extra at the ends and then also another length for the matching panel if there is one. Roll these lengths on to temporary cardboard tubes for easy handling.

Deciding whether to apply dry or wet is a personal thing based on experience.
If it's a complex part, I might fit and lay the glass on dry and then work to get the resin to penetrate. But personally, I generally prefer to brush (small areas) or roll (large sheets) on the resin first and then lay the cloth into the wet resin. The disadvantage of this approach is that it's harder to maneuver the glass into the exact position but the advantage is that it wets-through quicker and perhaps more completely.

If rolling, use a 3" (75 mm) firm, thin foam roller. Initially mix up 4.5 oz of resin for each sq-yd of glass cloth you will be putting down but if you're working alone, I would suggest to not plan on doing more than about 20 ft² (2 m²) at one time or you may find you are wasting epoxy as it could be setting off before the cloth has been wetted out. For really complex areas, even 1 m² can be a real challenge. Make sure the resin and hardener are well mixed and use the slow hardener unless you have to do this in very cool weather.

Oct 2011: More recently, as I've gotten back into boatbuilding after a fairly long break, I've used a faster and more controlled way to lay down the first resin and this is now documented in the article Sheathing 101, so consider this as a good option or update where applicable—particularly for flat surfaces.

For a large area, like the inner side panel for a W17 ama, mix about 10 oz of epoxy and pour this down the middle, quickly pushing it over the whole area with the roller. If you put some pressure on the roller it will barely roll and this works well for pushing the epoxy to wherever it's needed.

Once spread out over the whole area, use the roller more lightly to spread the epoxy as evenly as possible and promptly lay the glass cloth on to the wet surface. After making sure the cloth is covering all the area, I suggest to wet out just the center portion for a good wet grip and then, using the spare material at each end, pull out the cloth so that the strands become pretty straight. If the cloth is being added for strength, straight warp and weft strands ARE important. If it's just to hold an epoxy coating, it's far less important. Light cloths have a tendency to lift with the passing roller, so work with a plastic spreader or so-called, 'squeegee'.

Pull the squeegee from the center towards the edge, mostly using an angle of 30–45 degrees from the center of the panel. With the right pressure, this will spread the resin evenly and pull out any bubbles of cloth.

I prefer a small squeegee of say 5" (130 mm) width as it stays straighter than a wider one.

If the roll of cloth has a stiff, bonded edge, I would strongly suggest to cut that off before wetting out, as it has a tendency to lift the adjacent glass off the plywood at the edge, even if the bonded edge is not actually over the plywood being sheathed.

Once complete, do not move a flexible panel until all is cured or you will create many more cloth bubbles. Decide where you want to leave it before leveling out the cloth.

One of the most complex W17 parts to sheath is the central web and dagger board case.
It might well take you 1.5 hours per side if it already has its small ledges in place to carry the cross beams as this one does. When sheathing, you will often see grey areas amongst the dark areas that are more fully saturated. This generally indicates that there's still air under the cloth and this needs to be worked out. There are several steps to achieve this. First, pull the cloth in the direction of the glass strands to make it as flat as possible.

You can also hold one end down with say a small brush and then pull the surface with the squeegee by stroking from the brush, away to the other edge. Then squeegee at 90 degrees to that direction. Most of the grey should then be gone, but you may still find a few smaller grey spots. Probably the now wet cloth is preventing the air from getting out so I find it effective to 'stipple' them with a short (1 cm) stiff brush—repeatedly tapping this vertically down on each spot from about 25 mm height. You'll sound like a woodpecker calling his mate but this generally works well for me. Working with the corner of a squeegee works also. Then pass the squeegee edge over the surface periodically and you'll soon have a clear, dark, even surface—but this is not a job that's quickly done, so plan to be patient.

After this first coating, the surface should appear matte everywhere. If it's already glossy then you've used too much resin. Regardless, sand the surface lightly before applying a thin second and sometimes third coat of resin. Eventually, all exterior coated surfaces will need to be painted as epoxy has poor resistance to ultra-violet rays.

As for most work associated with curing resins, choosing the right time to trim the rough fiberglass edges can make it very easy. Sharpen a steak knife (on the back edge of the serrations) and use it to shear off the excess within an hour after the epoxy has first cured.
Much easier than waiting a day and having to use a chisel ;-)


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