This is catching the MOAM's attention today….. the row of filler tubs on the shelf. On the Chamberlain Marblehead gunning dory I've got four or five of them going at once, and they all look like the same white-ish powder in a cup. They are not the same. One makes a joint you could hang the boat from. Another makes a putty that sands like a dream and has no business in a joint. Grab the wrong tub and you find out later, usually the hard way. So here's how I keep them straight: pick the filler for the job, then thicken it to the consistency the job needs.
Why it matters: WEST SYSTEM splits its fillers into two families. Adhesive (high-density) fillers, 404, 406, 403 and 405, cure to "a strong, hard-to-sand" epoxy plastic for bonding, fillets and hardware. Fairing (low-density) fillers, 407 and 410, cure to a "light, easily sandable material" for cosmetic work. Inside each family the fillers aren't interchangeable either. WEST's own selection guide rates each one for hardware bonding, general bonding, fillets, laminating and fairing, and the ratings move around. 404 is top dog for hardware. 406 is top for fillets. 403 is top for laminating. 410 gets no stars for anything but fairing. And the amount of filler matters as much as which one: the same cup goes from syrup to catsup to mayonnaise to peanut butter as you add powder, and each of those has its own job.
The fillers, in plain English (descriptions from WEST SYSTEM and their Epoxyworks magazine)
- 404 High-Density: a high-strength fibrous mineral. Developed for hardware bonding where high cyclic loads are expected. Cures off-white. The hardest of the bunch to sand.
- 406 Colloidal Silica: fumed amorphous silica (Epoxyworks notes it contains no crystalline silica). Controls viscosity and keeps epoxy from running out of vertical and overhead joints. Makes a smooth mix for bonding and fillets. WEST calls it their most versatile filler. Cures off-white.
- 403 Microfibers: mostly cotton flock. Thickens fast, grainy "like oatmeal" when mixed, and it's WEST's go-to for bonding wood: good gap filling while still wetting into the wood fibers. Cures off-white.
- 405 Filleting Blend: the wood-toned one, for glue joints and fillets on naturally finished wood. Cures dark brown and can be used to shade the other fillers.
- 407 Low-Density: microballoon-based fairing filler. Easy to sand or carve, reasonably strong for its weight. Cures dark red/brown.
- 410 Microlight: the lightest and easiest-sanding. Big fairing jobs only. WEST doesn't recommend it under dark paint or on surfaces that get hot. Cures light tan.
Generic wood flour from other suppliers works much like 405: a wood-colored structural filler for fillets and joints, often blended with silica to stop sag (TotalBoat, Jamestown Distributors).
Materials
- Epoxy resin and its matching hardener
- Adhesive fillers: 406 Colloidal Silica and 403 Microfibers cover most of it; 404 for hardware; 405 (or wood flour) for bright work
- A fairing filler (410 or 407), kept on its own shelf
- Clean mixing cups and stiff mixing sticks
- Gloves, safety glasses and a dust mask while you're handling the powders
- A scrap of plywood stood on edge for a consistency test
Tools
- Disposable brush or small roller for the neat wet-out coat
- Plastic spreader or putty knife
- Syringe for injecting catsup-thick mix into tight spots
- A labeled shelf or bin: adhesive fillers on one side, fairing fillers on the other
Steps
- Name the job before you open a tub. Is it a joint that carries load, a fillet, a bolt or fitting, a laminate, or a low spot you'll sand fair? Strength jobs get an adhesive filler. Sanding jobs get a fairing filler. That one decision keeps most mistakes off the boat.
- Pick the filler off the matrix. The plan above is WEST's selection guide in one chart. Glue joints, plank laps and scarfs: 403 or 406 (403 is the one WEST points at for wood). Bolts and hardware under load: 404, with 406 as the alternative. Fillets you'll paint: 406, the smoothest filleting mix. Fillets on bright wood: 405. Fairing: 410 or 407, and fairing got its own tip back in October, so I won't repeat it here.
- Mix the resin and hardener completely first. This is WEST's two-step rule for fillers: mix resin and hardener thoroughly, then add filler. Never stir powder into resin and then add hardener. Start with a small batch and leave room for the filler. For thick mixes, WEST says don't fill the cup more than a third full of epoxy before you add powder.
- Add filler a little at a time. Stir in small amounts until you reach the consistency you want, and get all of it blended before you apply it. Stir gently so the powder doesn't puff up into the air. There's no recipe to measure; WEST says to judge it by eye.
- Hit the right consistency for the job. WEST's four:
- Syrup (unthickened): drips off a vertical surface. For coating, wetting out before bonding and fiberglass.
- Catsup (slightly thickened): sags down a vertical surface. For laminating flat panels and injecting with a syringe.
- Mayonnaise (moderately thickened): clings to a vertical surface, peaks fall over. For general bonding, filleting and hardware bonding.
- Peanut butter (maximum thickness): clings, peaks stand up. For gap filling, fillets, fairing and bonding uneven surfaces.
Not sure? Put a dab on the scrap of plywood standing on edge and watch what it does.
- For strength, don't overload it. WEST: add only enough filler to bridge the gaps without the mix sagging or running out of the joint. A small amount should squeeze out when you clamp. A stiffer glue mix isn't a stronger glue mix.
- Wet out first, then the thickened mix. On wood, WEST's preferred method is two-step bonding: brush neat epoxy on both faces so it soaks into the pores and end grain, then put the thickened mix on one face. You can add the filler to what's left of the wet-out batch, so mix enough for both steps. WEST says the one-step shortcut (thickened mix straight onto bare wood) isn't for highly loaded joints, especially end grain or other porous surfaces.
- Blend when one filler isn't quite right. WEST says fillers can be blended for in-between handling. A little 406 stiffens a mix that wants to sag, and WEST uses it to improve the consistency of other mixes. 405 shades a mix darker. For bright work, Epoxyworks suggests thickening with 406 and adding a little sanding dust from the same wood to match the color.
MOAM's shop note: Color is the one people forget until the varnish goes on. 404, 406 and 403 all cure off-white. On a painted hull nobody cares. On bright work, an off-white glue line or fillet shows like a chalk line on mahogany. On the dory, anything that'll be varnished gets 405 or 406 tinted with sanding dust from the same plank, and I test the color on a scrap first. The other thing: keep the fairing tubs on a different shelf from the glue tubs. 410 in a structural joint is a mistake you won't see until the boat's working in a chop off Tinker's Island.
Sources: WEST SYSTEM, Filler Selection Guide and individual filler pages (403, 404, 405, 406, 407, 410); WEST SYSTEM User Manual & Product Guide (2020 US edition: fillers, four consistencies, two-step filler rule, one-third-full cup, two-step and single-step bonding, hardware bonding); WEST SYSTEM User Manual (March 2025 edition: filler characteristics ratings for sanding); Epoxyworks, "The Basics of WEST SYSTEM Epoxy Resin and Hardener" (what each filler is made of, two-step bonding, 406 plus sanding dust for color) and "Furniture Building Tips"; TotalBoat, "Epoxy Fillers Guide"; Jamestown Distributors, "Which filler should I use for my epoxy project?" Ratings are WEST's for WEST products; other brands' fillers differ, so check your own maker's chart.
MOAM Out.
