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Once you’ve tried filleting, you will find any number of applications for this technique, but it is used most extensively in Stitch-N-Glue construction. A fillet is a radiused bead of resin putty at the inside corner of wood junctions. Fillets are commonly applied where plywood parts join (such as planking junctions), where bulkheads mate to the hull, where cockpit soles mate to bulkheads, and also for aesthetic reasons. Parts can be bonded with fillets on one or both sides. Structural fillets are almost always reinforced with fiberglass.

Filleting Supplies

Sheathing material will tend to pull away from 90° corners as the cloth is worked, causing air entrapment. Should the fiberglass surface be broken, water could travel along this void and create a potential for dry rot. A fillet also increases the bonding area at the joint and strengthens it. The fillet material is a mixture of activated “POXY-SHIELD” and one or more of our filler materials, such as Microspheres, and/or Silica. Fiber Filler tends to be harder to work but can be used. We use a combination of Silica for its thixotophic (non-sagging) characteristics, and Microspheres for its ease of sanding.

How Big Are Fillets?

How big are fillets? This will depend on the angle of the junction. On a typical Stitch-N-Glue boat, the planking chine junction may vary from close to 90° to almost a straight line. In this instance the 90° fillet will usually be thicker; and the flatter angle, a larger radius, but thinner. Since structural fillets depend on the combination of the fillet and the cloth, the fillet contour should be large enough to eliminate voids in the corner, but narrow enough to allow the cloth to lap onto the wood and not just the fillet. With a lightweight fiberglass cloth, a 3/4″ radius fillet would be enough for the typical 6 oz. to 7 1/2 oz. weight to conform to. If the fillet is not primarily used for structural purposes, the radius is strictly what is required to work.

What consistency should the fillet mixture be? Basically, thick enough to do the job, but a consistency that will hang on your mixing stick is probably about right. The exact proportions of fillers used can vary with temperature (warmer temperature = “runnier” mixture), type of filler materials added and application. We have found that a ratio of 2 to 2 1/2 parts of filler mixture to 1 part of resin/hardener mix is about right. If you use cellulose or cotton fibers, start with a smaller proportion of filler (1 to 1 1/2 filler to 1 part resin/hardener) and add additional filler until desired consistency obtained. Do not add so much filler that the mixture becomes “dry”; this will make the fillet weak. When mixing, resin and hardener should be mixed first, then the fillers added. When you find the combination of fillers that best suit your needs we suggest that the filler combination be pre-mixed so it can be more easily measured and added to the epoxy.

Start filleting in small increments until you get a feel for the process. Some practice is necessary to make nice, smooth fillets, and much depends on the “right” consistency of the mix. All surfaces of joining pieces must be coated first with activated “POXY-SHIELD” or the resin in the fillet mix will be drawn into the wood. If pre-coating was done and allowed to cure, make sure the surfaces are lightly sanded before applying the fillet.

To apply fillets you will need various radii applicators that you can make yourself. Any round object can be used to form fillets, including tongue depressors, plastic lids, spoons, etc., but cutting your own from wood or flexible squeegees allows you to adapt the tool to the work rather than vice versa. Applications such as Stitch-N-Glue construction will require several different radii depending on the joining angles. Test tools prior to filleting by sliding them along joints to see that they give the radius you need at various junctions. Filleting Tool

To apply fillets, “glop” the filleting mixture in place then use your filleting tool to gradually spread the material and to pick up excess or add additional material.

Save yourself a lot of sanding by doing a good clean-up after applying fillets. Use a putty knife or comparable tool to remove ridges and other spots of excess fillet mixture. Once cured, the fillet can be sanded smooth. Un-reinforced fillets should be covered with at least one coat of “POXY-SHIELD”.

Your Thoughts?

2 Responses to Filleting: What Is It? Where Can You Use It?

  1. Douglas Wade says:

    The addition of silica is an axcellent idea. I place the mixture into a zip lock bag and then cut a corner off. A bead of exact size can then be squeezed out just like decorating a cake. With the amount of compound carefully controlled clean up after is very easy.

  2. Robert Rowe says:

    Great timing for me as I now have my boat based on the Ski King design turned over and remembering those juctions that could not be filled (or fillieted) in the upside down orientation – it all runs out!

    So next step will be basd on Glen’s nots (paper) – thanks

    Robert Rowe – Goolwa South Australia

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