Wednesday 3 June 2020

Rear Cabin Wall, Seatback Supports & Topsides | Fillets

When the tack welds were fully cured I pulled all the stitches and prepared to apply fillets to the joints.

Now, any readers of earlier posts here will know that epoxy fillets are my nemesis and I can never summon up any measure of enthusiasm for working on them. Other than for finishing them and moving on to something more satisfying.

But based on my previous experience I now have a method and an armoury for tackling the dreaded epoxy resin fillet. This is how it goes:

Use blue tape to mark out fillets on flat surfaces

No amount of freehand work will yield a neat finish on a very slightly curved surface, such as the topside to side panel joint.

It will just look like a child has been playing with mud pies. Use blue tape to get neat edges, and lots of it.

Don't expect the first application to leave a smooth finish

The wood flour fillet mixture has to be quite thick for it to stick to the surface of the hull, and hoping for a miracle during application isn't going to have a happy ending.

The epoxy will cure with a rough and spiky surface, like cast concrete, and that is just the way it is.

A thinner mixture is much easier to work with, but it sags and falls off vertical surfaces and that is not the way to go.

Use industrial abrasive finishers to dress the rough fillet

These include carbide burrs and abrasive wheels used in a corded or cordless drill to rapidly remove the rough surface and achieve a good profile.

I call this dressing the fillet.

Sand dressed fillets to a good surface

A dressed fillet can be sanded to a reasonably good surface, either by hand or with a finishing sander. Use a P80 grit.

Apply a coat of fillet mix as a fairing compound

Apply a coat of thinnish wood flour fillet mixture to the fillet, effectively as a fairing compound, and let it cure completely.

Sand to a good finish

It is possible to achieve a good, smooth surface to the fillet by sanding, again either by hand or with a finishing sander and using a P80 grit.

This will be good enough for the application of fibreglass cloth, and after two or three coats of clear resin have been applied to the cloth and sanded the fillet will be invisible.

But if a truly smooth surface is required for e.g. a paint finish, then apply some fairing compound made from glass micro balloons (use the phenol variety below the water line) and sand by hand to as fine a finish as you wish. I have finished fillets to a P220 grit for paint in this way, and they look great.

So … that's a long winded way of saying that I'm not going to bore the reader with a long catalogue of photos of the interminable and tedious making of fillets.

If you have a look at earlier posts you will see all of the above activities in action. Instead, here are some selected highlights to show you what I mean.

This is a fillet on the forward deck being dressed with a carbide burr and a small but powerful corded drill.

And here are the forward deck fillets after sanding, fairing, and final sanding by hand and the finishing sander.

These fillets will be covered with fibreglass cloth sometime soon.

Here is the taped out starboard topside join in the cockpit, ready for fillets to be applied.

And here is the fillet between the rear cabin wall and the cockpit deck, being dressed with a Spiraband abrasive wheel and a cordless drill.

And this is the same fillet, after fairing and sanding with the finishing sander. It's now ready for the application of heavy duty fibreglass tape.

And last but not least, a reward for the boatbuilder after two days of purgatory, sanding fillets ...


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