Thickened epoxy: glue, fillet and fairing mixes explained

Almost every plywood boat built at home is held together by the same stuff: epoxy with something stirred into it. Glue a scarf, run a fillet along a seam, fill a screw hole, fair a hollow in the topsides, and each time the job is really the same question. How thick should this mix be, and what goes in it?

Get it right and the joints are stronger than the wood around them. Get it wrong and you get glue that runs out of the joint overnight, fillets that sag into lumps, or filler so hard you spend a weekend sanding it back. This post covers the mixes a first-time builder actually needs, how to tell them apart, and how to get each one where it belongs.

What thickened epoxy is

Thickened epoxy is ordinary mixed resin and hardener with a powder filler added until it stops flowing and holds its shape. The resin still does the bonding. The filler changes how the mix behaves: whether it stays in a vertical joint, how hard it cures, how easily it sands and what color it ends up.

Two rules sit underneath everything else:

1. Mix the resin and hardener completely first, then add filler. Filler stirred into unmixed resin hides the streaks, and a patch of uncured epoxy in the middle of a joint is very hard to find later.
2. Thicken to suit the job, not the habit. The right consistency for glue is wrong for a fillet, and the right one for a fillet is wrong for fairing.

The three consistencies

Builders and the epoxy makers describe the mixes by kitchen comparisons, and they are worth learning because every set of plans and every forum thread uses them.

• Ketchup. Still flows, but slowly. Used where you want the mix to soak into wood and fill a joint without leaving a bead, such as wetting out a tight-fitting scarf or a puzzle joint.
• Mayonnaise. Holds a soft peak and does not run off a vertical surface. This is the general purpose glue mix for most structural bonding: frames to planks, butt blocks, gussets.
• Peanut butter. Stiff enough to stay exactly where you put it, even overhead. This is the fillet mix, and the one for filling gaps where parts do not quite meet.

Mix a little stiffer than you think. A mix that looks right in the cup will slump once it warms up in the joint, and a cold shop makes it easy to under-thicken because everything looks firmer than it will be at curing temperature.

Which filler for which job

The filler matters as much as the consistency. The WEST SYSTEM filler selection guide is the clearest single reference on this, and most other epoxy brands sell close equivalents under their own names. Broadly they fall into two groups.

High-density fillers, for strength. Colloidal silica, cotton or cellulose fibers, and the high-density adhesive blends make a mix that cures hard and strong. Use them for glue joints, structural fillets and anything that carries load. Colloidal silica on its own thickens very efficiently and gives a smooth, creamy mix, but it cures hard and sands poorly, so keep it off surfaces you will need to fair.

Low-density fillers, for fairing. Microballoons and similar lightweight blends make a mix that is easy to spread and much easier to sand. Use them to fill hollows, screw holes and the weave of fiberglass before paint. Never use them where a joint needs strength.

Wood flour sits in between. It makes a decent fillet that is easy to shape, and it cures to a wood color, which is why many builders use it for fillets that will sit under varnish. A blend of wood flour with a little silica is a common fillet mix for exactly that reason: the flour gives color and body, the silica keeps it from sagging.

If you only buy two fillers for your first boat, buy one structural filler and one fairing filler, and keep them in clearly marked containers. Mixing up the two is one of the few mistakes that is hard to undo.

Getting it into the joint

How you apply the mix matters almost as much as what is in it. A few methods that work:

1. Wet the joint first. Brush plain mixed epoxy onto both faces of a glue joint and let it soak for a few minutes before applying the thickened mix. Bare plywood will otherwise wick resin out of your glue and leave a starved, weak joint.
2. Use a bag for long seams. A disposable piping bag or a heavy zip-lock bag with one corner cut off lays a clean, even bead along a seam far faster than a stick. Pastry bags from a kitchen shop work well and cost very little.
3. Shape fillets with a rounded tool. A plastic spoon, a tongue depressor with a rounded end, or a filleting tool sized to the radius you want. Draw it along the bead at a steady angle, and scrape the squeeze-out on either side away straight after.
4. Clean up while it is soft. Thickened epoxy that has cured in the wrong place has to be sanded off. A stick and a rag dampened with a little vinegar or denatured alcohol does the job in seconds while the mix is still wet.
5. Tape over wet fillets. Where the plans call for glass tape over a fillet, lay it on while the fillet is still soft or barely firm. The tape beds into the fillet and the two cure as one piece, with no sanding in between.

Mistakes worth avoiding

Most problems with thickened epoxy come from a short list.

• Mixing too much at once. Epoxy cures faster in a deep cup because it heats itself. Mix small batches and spread them out on a flat tray if you need more working time.
• Starved joints. Clamping a glue joint so hard that all the mix squeezes out leaves almost nothing in it. Clamp just firmly enough to close the joint and leave a small, even bead of squeeze-out.
• Fillets that are too big. A larger fillet is not a stronger boat. Follow the radius in the plans. Oversized fillets add weight and take longer to sand.
• Sanding before full cure. Thickened epoxy that is firm but not cured clogs paper and tears. Give it the time the epoxy maker states for your temperature.
• Working in a cold shop. Below the temperature range on the tin, epoxy cures slowly and sometimes never fully. Warm the resin and the shop, not just the boat.

None of these will ruin a boat on their own, but each one costs hours. The logs on the builds page are full of builders who learned them the slow way, and the stats page shows the real hours those builds took. Fairing and finishing is where a poor filler choice comes back to bite.

The build on this page

The Northeaster Dory on this page is being put together in a single garage in Cologne, and it is a good example of all of this in practice. It is a Chesapeake Light Craft kit designed by John C. Harris: a 17 foot lapstrake dory with 6mm okoume planking and a 9mm bottom, glassed on both sides. Every one of those lapped planks, puzzle joints and bulkheads is bonded with thickened epoxy, and the builder's latest entry describes laying the glue with a pastry bag with a promise of cake once the dory is finished.

Where to go from here

Thickened epoxy is the skill you will use more than any other on a plywood boat, and it rewards practice on scrap before it goes on the hull. Mix a cup of each consistency, glue up a few offcuts, run a fillet along a scrap corner, then break them apart once cured and see where they fail.

If you are choosing a boat to start on, the designs catalog lists the plans and kits builders on this site have used, with the construction method for each. And when you start, log the build. Your notes on what mix worked in your shop at your temperature will be the most useful thing you write down.