Two tubs of powder on the shelf, and they are not the same thing….. It's an easy grab to get wrong. Fill a gappy plank lap with fairing compound because it "spreads so nice" and you've got a weak joint. Fair a whole topside with colloidal silica and you'll spend two weekends sanding it like you're trying to wear through a granite curb. Both mixes start as epoxy and a white-ish powder. What they turn into is completely different. On a boat like the Chamberlain Marblehead gunning dory I'm building, it pays to keep the two families on separate shelves. Here's the rule: strength goes in the joint, fairing goes on the skin.
Why it matters: WEST SYSTEM sorts its fillers into two camps. Adhesive (high-density) fillers, 404, 406, 403 and 405, cure to "a strong, hard-to-sand cured epoxy plastic" for bonding, fillets and hardware. Fairing (low-density) fillers, 407 and 410, cure to a "light, easily sandable material" for cosmetic work: shaping, filling and fairing. On WEST's own selection chart, 410 gets no stars at all for general bonding or fillets. System Three says the same thing from the other side of the street: microballoons and microspheres "should not be used in adhesive formulations as they reduce tensile strength." So a fairing putty in a structural joint is a weak joint, and an adhesive filler on the topsides is a hard lump that won't sand at the same rate as the wood and fairing around it. Pick the powder for the job, not for whichever tub is closest.
What goes where on a hull
- Plank laps, scarfs, chine and bottom seams, transom and stem bonds: an adhesive filler. 406 colloidal silica or 403 microfibers at mayonnaise consistency for general bonding (WEST). 403 is the one WEST singles out for wood.
- Fillets (bulkheads, frames, inside corners): 406 or 405 at peanut butter consistency. 405 is WEST's wood-toned choice for fillets and glue joints on bright-finished wood. (Full fillet how-to is in my Sept. 25 tip.)
- Bolts and hardware under load (oarlock sockets, a towing eye): 404 High-Density, which WEST rates highest for hardware bonding where cyclic loads are expected.
- Low spots, dings, staple and fastener holes, glass-tape edges on a painted hull: a fairing filler. 410 Microlight is the lightest and easiest to sand; WEST says it's about 25% easier to sand than 407 and about three times lighter.
- Under dark paint, on sun-baked topsides, decks and thwarts that get walked on, or a thicker fill: 407 Low-Density. WEST doesn't recommend 410 under dark paint or on surfaces that get hot, and calls 407 the better choice for thicker fills or where you need impact or abrasion resistance.
- Bright (varnished) work: leave the fairing fillers on the shelf. 407 cures dark red-brown and 410 cures light tan (WEST), so they're for painted surfaces. Fair bright wood with a plane and a scraper.
Materials
- Epoxy resin and hardener (WEST 105 with 205 Fast or 206 Slow; pick by shop temperature, see the shop note)
- Adhesive filler for joints: 406 colloidal silica and/or 403 microfibers (404 for hardware)
- Fairing filler: 410 Microlight for big, light-colored or underwater areas; 407 Low-Density under dark paint or where it'll take a beating
- Carpenter's chalk or a soft pencil for finding highs and lows
- Release fabric or plastic (optional, for batten guides)
- Aluminum oxide sandpaper, 50/60 and 80 grit (WEST prefers aluminum oxide for cured fairing material)
Tools
- A long, knot-free batten (wood, plastic or metal)
- A longboard: a flexible fairing board with handles, long enough to bridge the lows (on a 40-foot hull that's 3 to 6 feet; on the dory, a couple of feet for the planks)
- Plastic spreaders, one with notches cut in it, and a wide smooth knife
- Mixing pots, sticks, and a scrap of plywood to spread the batch out on
- A card scraper for drips and ridges
- A real dust mask or respirator and eye protection (WEST calls fairing dust one of the most hazardous parts of the job)
Steps: fairing a low spot the right way
- Find the lows before you mix anything. Rub a chalked batten over the area; the chalk lands on the high spots. Or scribble pencil all over it in a few directions and take a pass with the longboard: what's left of the pencil is low (Epoxyworks). Decide whether you're filling the lows, knocking down the highs, or both. Then sand the whole area with 80 grit so the compound has something to grab.
- Clean it and wet it out. If you're going over cured epoxy, wash off the amine blush first with water and a Scotch-Brite-type pad, dry it with plain paper towels, and sand any shiny spots (WEST). Bare wood gets a coat of neat epoxy so the wood doesn't suck the resin out of the fairing mix. Sanded epoxy doesn't need a wet-out. On the topsides, let the wet-out go tacky or gel before you trowel, or the compound slides right off (WEST).
- Mix resin and hardener first, then the powder. Fill your pot no more than a quarter full of mixed epoxy to leave room (WEST). 407 roughly doubles the volume of the batch and 410 makes it about two and a half times bigger (Epoxyworks). Stir in filler until it's a stiff, non-sagging peanut butter. The more filler, the easier it sands; too thin and it sags and sands harder (WEST). Spread the batch on a scrap of plywood to buy working time.
- Trowel it on a little proud. Work it into the low with a plastic spreader and leave it slightly above the surrounding surface, so you sand down to fair instead of filling twice. On a big area, drag a batten across the fill with a slight Z motion. For whole topsides, one pro shop uses a notched trowel ground to leave ribbons about 1/8" deep and 1/4" wide, then fills the valleys later (WoodenBoat). Anything deeper than 1/2" goes on in two or more layers, the second after the first has partly cured, so it doesn't cook itself (WEST).
- Let it cure all the way. Sand when it's solid: you can't dent it with your thumbnail (WEST). At 72°F, thin films of 105/205 get there in about 6 to 8 hours and 105/206 in 10 to 15 hours. In a cold shop it's longer, so wait for the thumbnail test, not the clock. Wash off any blush before you sand, because it clogs paper.
- Longboard it fair. Start with 50 or 60 grit on the longboard to take down the fill over a broad area, then go to 80 when you're close (WEST). Change direction often, swinging the board through about 0°, 45° and 90° so you don't sand a valley in. No finer than 80 at this stage. Check with the batten: it should touch everywhere with no gaps. Fill the leftover lows and repeat. You don't need another wet-out unless you sanded through into the original surface.
- Seal it before paint. Fairing compound is full of broken microballoons, and paint will soak into them and look ragged (System Three). WEST says an epoxy barrier coat is required over sanded 410 before any solvent-based paint (solvents can cause print-through) and recommended over 407. For a moisture barrier on exterior surfaces, WEST's fairing manual calls for at least three coats, about 10 mils, and up to six coats, about 20 mils, for more protection. Recoat when the last coat still takes a fingerprint but doesn't come off on your finger. After the last coat cures, wash and sand it for the paint you're using.
MOAM's shop note: On a lapstrake boat like the dory, the laps are the look. Fairing compound is for dings, hollows and fastener holes inside a plank, not for filling the landings flush. And where a hole sits in a structural spot, like an old screw hole through a lap, fill it most of the way with a 406 bonding mix and put only the last skin of 410 on top, so the strength is underneath and the easy sanding is on the surface. The other thing that bites New England builders this time of year: WEST lists a 60°F minimum for 206 Slow hardener, while 205 Fast works down to 40°F. In an unheated shop in late October, that's the difference between fairing that's ready to sand Saturday morning and a tacky mess you're still poking at on Sunday.
Numbers here come from WEST SYSTEM / Epoxyworks, System Three and WoodenBoat; where a figure is one shop's practice, I've said whose.
Sources: WEST SYSTEM, Filler Selection Guide, Fairing, 410 Microlight, Final Fairing & Finishing manual and User Manual & Product Guide; Epoxyworks, Epoxy Fairing Basics and Preparing to Fair; System Three, Bonding with Epoxy and Filleting, Fairing and Molding; WoodenBoat, Andy Giblin, Fairing with Thickened Epoxy.
MOAM Out.
Tools for this tip
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- West System 406 Colloidal Silica filler (1.7 oz): the bonding and filleting filler: laps, chine seams, screw holes that carry load
- WEST SYSTEM 410 Microlight fairing filler (5 oz): the easiest-sanding fairing filler for big, light-colored or below-the-waterline areas
- WEST SYSTEM 407-5 Low-Density fairing filler (4 oz): the tougher fairing filler for dark paint, sun-hot topsides and decks that get walked on
- West System 105 Resin + 206 Slow Hardener bundle with 300 Mini Pumps: wet-out, the fairing mix itself, and the seal coats over the sanded fairing
- The Gougeon Brothers on Boat Construction (book): the Gougeon brothers' book on fairing, coating and building a hull with epoxy
Use whatever you already have in the shop; these are just the ones that fit this job.





















