Filleting is easy, isn’t it? Just drag the stick along the joint and presto, job done.

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As anyone who has actually tried it will know, that’s the approach of the blinkered enthusiast.

The reality can be a sagging mess which transfers itself everywhere but to the gluing area, leaving a huge clearing-up job.

However, with the right tools, the right mix and a bit of technique it is possible to produce neat coves with minimal mess and waste.

Our new project will need hundreds of fillet joints, so we asked Hamish Cook of Wessex Resins to visit and demonstrate their methods, using their WEST SYSTEM epoxy product.

Preparation

Preparation is essential to ensure a good bond. Epoxy forms a mechanical bond by filling tiny cracks and crevices in the substrate, so if you’re working with a smooth surface such as GRP you’ll need to key it by roughly sanding the area.

Absorbent materials such as wood tend not to need keying, but it’s essential to make sure the gluing areas are clean, dry and free of dust before starting. To avoid the substrate absorbing the resin from your mix, it’s also good practice to prime the area with neat epoxy.

Ideally this should be left to go tacky before filleting (up to around an hour depending on temperature), preventing the primer resin from diluting the thickened epoxy mix while still allowing the two to bond chemically. This step can be omitted, but you may need to allow for slight dilution by using a thicker mix.

Mixing

You’ll probably already have some epoxy mixed from priming the joint, so it’s now time to add some filler. West make this easy by providing a ready-mixed filleting blend, but if you don’t have this you can use a mix of microfibres and colloidal silica.

You’re aiming for a mix the consistency of peanut butter, or slightly thicker if you haven’t been able to let the priming resin go tacky. West say that a heaped scoop of filleting blend mixed with a single (one pump each of resin and hardener) mix of epoxy should take you straight to the correct consistency, but it’s better to err on the side of caution and mix by eye.

As you add filler to the mix, you can see it thicken and assess its consistency. As you reach the correct consistency the mix will lose its gloss – you want to be just on the cusp of this transition. We found it pays to slightly overstep this point if filleting on to wet priming resin, but to stay shy of it if the resin
has gone tacky.

Top tip: to avoid clouds of dust as you mix in the filler, try a cutting motion rather than simply stirring it in.

 

 

Filleting

Not all sticks are equal. It might sound obvious, but your stick needs to be shaped to give the required cove shape. Conventional lolly sticks are usually too small, but tongue depressers or Magnum sticks work well.

West also do their own plastic sticks, which work well and have a handy spatula on the other end for removing the excess. At the time of writing, you could pick up 25 sticks for £10 on eBay, and if you wipe them off before the epoxy cures they can be used repeatedly.

You can either roughly push the epoxy into place using the stick before shaping, or for a more controlled application use a piping bag. This is a pleasingly simple and tidy process – once you have made a mix, push it into a plastic bag, snip off the corner and pipe the epoxy into place like icing. The only problem is that you need to work fast – the dense mass of epoxy and the heat of your hands serve to start it curing more quickly than if spread around the mixing pot.

Once you have the epoxy in position, use the stick at an angle of about 45° to shape it and push it deep into the joint. Too shallow an angle won’t apply enough pressure, but if the stick is too near vertical it will pull the epoxy away from the joint.

As you work the fillet, any excess will pile up at the sides. To remove the excess, a thin plastic spatula will lift it away. Assuming the epoxy is clean, it can be kept aside to re-use.

Finishing

You could leave your fillet at this point, but there are two disadvantages. Firstly, the thick epoxy mix can look a little rough and, secondly, the exposed epoxy will ‘blush’.

Amine blush is a process by which epoxy forms a waxy top layer as it cures, which must be removed before paint or further epoxy is applied.

Blush can be removed with acetone or by sanding but an easier method, which also leaves a tidy finish, is to use peel ply. This is a synthetic cloth which can be applied to wet epoxy: the epoxy doesn’t stick to it, so it can be peeled away after the epoxy has cured, taking the blush with it and leaving a slightly textured surface.

Peel ply is available as a tape, which is ideal for filleting. Just cut a length and lay it over the fillet, then rub a thumb or clean filleting stick over it to encourage the epoxy to wet it out. Pay particular attention to the edges, as the peel ply helps to spread the edge of the fillet to give a gentler transition from the substrate behind.

It’s essential to make sure the peel ply is fully wetted out, as if there is any air underneath it the epoxy can still blush. If your mix is too dry to wet it out properly, a little unthickened epoxy stippled into the tape can save the day.

Once the epoxy has fully cured, simply peel off the peel ply and throw it away.

Taped joints

When used on timber the wood usually gives way before a fillet joint. A good way to spread the load on the joint and increase the bonding area is to add a layer of glass tape over your fillet.

This can either be done immediately after filleting while the epoxy is still wet, a little later while it is tacky, or after curing if the fillet is sanded or peel ply is used. Working wet risks destroying the shape of the fillet, so if you have time a two-stage process is more controllable.

Whatever you choose to do, use a brush to generously wet out the fillet and the areas either side where the tape will sit with unthickened epoxy, then lay the tape in place. A vigorous stippling action with the brush (somewhere between stabbing and brushing) should allow the epoxy to fully wet out the tape. If it doesn’t, use the brush to add a little more.

When fully wetted out, the tape will be nearly transparent, but you should still be able to see the texture of the weave. If you apply too much it will appear smooth, in which case it is possible for the tape to float away from the fillet and weaken the joint.

Most glass tape has a plain weave, with glass strands laid at right angles to each other. This adds more than enough strength for most applications and comes in a range of widths. However, it does have the disadvantage that only half the glass strands are taking load – those laid across the joint.

For yet stronger joints, biaxial tape has diagonally-laid strands which spread the load more efficiently between them.

Frame-to-keel joint on the boat – step by step

As published in the December 2016 issue of Practical Boat Owner magazine.