This is catching the MOAM's attention today….. how many good wooden boats get a fiberglass skin that's half stuck, half floating and full of little white dry spots. The Chamberlain Marblehead gunning dory is going to get cloth on her bottom, so before I mix a drop I went back to the people who make the stuff and wrote down how they say to do it. Here it is, the way I'll do it in my shop.
Why it matters: Glass cloth over wood is there for abrasion and impact, and to keep the wood under it sealed. It only does that job if every thread is soaked and the cloth is pressed tight to the wood. Too little epoxy and you get dry, white weave that's weak and wicks water. Too much and the cloth floats up off the wood on a puddle of resin, which is heavy, brittle and a pain to fair. Work it too hard and you whip in tiny air bubbles that show up forever under a clear finish.
The numbers, from the epoxy makers
- Cloth: System Three's guide says 6 oz cloth is enough for most surfaces, doubled in high-wear spots. That's what goes on most small wooden boats.
- Cut it oversize: WEST SYSTEM says to cut the cloth several inches larger than the surface on every side, and to overlap pieces about 2".
- Small batches: WEST says three or four pumps each of resin and hardener at a time. Small batches don't kick off in the cup while you're still spreading.
- How much epoxy: WEST's table puts 6 oz cloth plus two fill coats at about 1.61 fluid ounces of mixed epoxy per square foot, waste included. So 40 square feet of bottom is roughly 64 fluid ounces, half a gallon. Buy a little extra.
- Fill coats: two or three coats to fill the weave, so your final sanding doesn't cut into the cloth.
- Trim time: System Three says trim the overhang at the "green" stage, when it's pliable but not tacky unless you press hard. An Epoxyworks article puts the minimum time between coats at about 3 hours, or when you can trim the cloth without resin sticking to the knife, at room temperature. Cold shop, longer wait.
Materials
- 6 oz fiberglass cloth (plain weave), enough to cover with several inches spare all around
- Epoxy resin and hardener, with the maker's pumps
- Masking or duct tape, or staples, for sloped and vertical areas
- Mixing cups and sticks, nitrile gloves, and some paper towels
Tools
- Plastic squeegee (a flexible one, not a putty knife)
- Thin foam roller covers and a tray for vertical surfaces and fill coats
- Sharp scissors and a fresh utility or single-edge razor blade
- Metal straightedge for trimming overlaps
- Respirator for sanding later (see the 10/23 dust tip), and a fan or open door for ventilation
Steps: wetting out cloth the dry way
- Prep the wood like it's a glue joint. Fair, sand, vacuum and wipe it clean. Any epoxy already on there that's cured gets washed for blush (see the 10/24 tip) and sanded dull.
- Lay the cloth on dry. Unroll it, smooth it with your hands from the middle out, and let it hang several inches past every edge. Overlap pieces about 2". Tape or staple it on anything that slopes.
- Mix a small batch. Three or four pumps each. Mix it well, scraping the sides and bottom of the cup.
- Pour in the middle. Pour a small pool near the center of the cloth on a flat area. On vertical or steep spots, roll it on instead.
- Spread it out, gently. Push the epoxy from the pool into the dry cloth with the squeegee. Let it soak in. Don't try to force it through. The cloth goes clear when it's wet out; white means dry, so add a little more there. Over bare wood, leave enough for the wood to soak up too.
- Smooth wrinkles as you go. Work them out toward the edges. On a tight curve or corner, snip a dart with the scissors and lap it for now.
- Squeegee off the extra before it gels. Hold the squeegee low, almost flat, and pull slow, even, overlapping strokes. Take off the shiny excess that would let the cloth float, but don't press so hard you make dry spots. What you want is evenly clear cloth with the weave texture still showing.
- Keep going, batch by batch. Mix the next small batch and work into the wet edge of the last one.
- Trim at the green stage. When it's pliable but not tacky unless you press hard, run a fresh blade along the edge of the panel. To trim an overlap into a flush butt joint, cut down the middle of it against a straightedge, peel out both scraps, and re-wet the edge. Or leave the overlap for strength and fair it in later.
- Fill coats before full cure. Roll on thin fill coats as soon as the last one is tack-free. Two or three will fill the weave. If it sits past full cure, wash off the blush and sand before the next coat.
- Sand the epoxy, not the cloth. When the weave is filled and cured, sand for paint or varnish. If you see white fibers, you've gone through the epoxy into the cloth. Stop and recoat there.
On the dory: Her bottom is the part that hits the beach, the rocks and the trailer, so that's where the cloth goes. I'll lay it on dry, tape it along the chines so it can't creep, and work from the middle of the bottom out to the edges in small batches. It's going to be painted, so a few micro-bubbles won't show, but I'll still keep the squeegee work to a minimum. And I'll do it on a warm afternoon with the shop heater running, because a cold October shop in Marblehead slows the cure way down.
MOAM's shop note: The mistake I see most is the guy who dumps a big puddle and chases it all over the hull with the squeegee, back and forth, back and forth. Now the cloth's floating in one place, dry in another, and the whole thing is foamy. Small batches, pour it, spread it, let it soak, one or two passes with the squeegee held flat, and walk away. If it's clear and the weave still shows, you're done with that section.
The numbers here come from WEST SYSTEM's instructions and coverage table, System Three's fiberglassing guide, and an Epoxyworks article on glassing a hull. The 40-square-foot example is my own arithmetic from WEST's table. Your epoxy's own instructions win if they say something different, especially on cure times.
Sources: WEST SYSTEM, Applying Fiberglass Cloth & Tape and Estimating Epoxy Coverage for Fabric Application; System Three, Fiberglassing with System Three Epoxy; Epoxyworks, Fiberglassing a Woodstrip Hull.
MOAM Out.
