Blind Fillets: A step-by-step walk through

If you’ve done much reading about stitch-and-glue boat building, it’s likely that at some point you have come across someone stating that they used a “blind fillet” for some situation or another.

Assuming you’re already acquainted with stitch and glue boatbuilding methods, you likely already know that a fillet is a key structural transition between two parts of a boat that blends them seamlessly together.

A pronunciation note here for those who hear words in your head when you read: in boatbuilder-speak, the word fillet is commonly pronounced “fill it” (rather than “fill-ay” like a filet of meat). When I first started getting into boat building, it took me a while to get over “correcting” the pronunciation of fillet. I’ve now happily been in the fill-it camp for years.

I’m pretty accustomed to using fillets as a key structural component in my stitch and glue boat building career.

What does “blind” mean… is this some sort of next level test of skill? Once you’ve mastered filleting, your next task to master is to do it blindfolded? I’m here to tell you that the reality is much less dramatic and mysterious than that.

So what is this mythical “blind” fillet … and why is it blind?

A blind fillet is simply a fillet that you put into your project in advance of all the components being installed. For instance, let’s say you have a deck to install and want to have fillets on the bulkheads that support that deck. But once you have the deck on, there will be no feasible way to get in and apply the fillet. In this scenario, bonding your fillets to your bulkheads prior to installing the deck makes a ton of sense. But how do you do that?

The simple answer: you fake it. By that, I mean that you fake the deck. When the real deck is there in its entirety, it blocks the necessary access you’d need to make the fillet. So instead, we make a “stand in” for the deck that only comes into contact with the critical points along the bulkhead – a thinner, narrower object that will align properly, yet give us the necessary room for access.

With that temporary part in place, you can proceed to make your fillet–then when the temporary object is taken away, you’ve got a spot ready and waiting for the real part to be installed onto.

Disclaimer here: this is one take on what a blind fillet is. You may very well have a different understanding and likely a different method. I’d love to hear your feedback and see how you have put blind fillets to use on your projects.

How to Make a Blind Fillet: Step by Step

Recently when we were building our demo Scout Sailboat Kit for the Seattle Boat Show I had a good opportunity to use a blind fillet and document for you how it’s done. The Scout has a deck where there are points along the way where a blind fillet makes sense.

Step One: Create a “Fake” part using a substitute material

On our project, I cut some 1⁄4” thick plexiglass rips about 2-1⁄2” wide and long enough to span the full width of the bulkhead, and then some. You could use a different material that isn’t see-through, but I find it helpful. You could also use a strip of plywood or solid lumber that has a clean and joined face. Whatever you use, put some packing tape on it so your fillet doesn’t stick to it.

Step Two: Check placement and alignment of your Fake part

Figure out a way to clamp your “bit of deck” into place and be sure to check to make sure your piece is properly aligned with the plane where you expect the actual deck to be. By that I mean, make sure that the thing you are using to stand in for the deck is actually in the same position that the deck needs to be in. You don’t want your blind fillet to create a lump in your deck because it was proud.

Step Three: Make your fillet

Now, you’re ready to fillet, which is more or less the same process that you already know. With this bit of fake deck in play it’s a bit awkward, but it’s manageable. Baggie your fillet mix in, shape the fillet and clean up. Once cured, you just unclamp the plexiglass and remove it. The fillet left behind is now bonded to your project and develops the plane that your deck will need for proper support.

Step Four: Glue in your Real part

If you’re ready to glue your deck in place you could get to it right away if your epoxy is still chemically receptive. If it has fully cured, run some 80 grit sandpaper over the glue surface, vacuum and wipe with denatured alcohol and you’re ready to go. 

What You'll Need 

There are a couple of ways that folks go about making fillet mix. The first and easiest is to let someone else do it for you. System Three Ez Fillet is a no brainer. It's the same mix each time, no guessing if your fillets are going to sag or not. 

That being said, many prefer to mix their own thickeners into the basic epoxy resin/hardener mix. This gives you more control over the relative strength of the fillet, balanced against the "sand-ability" of the fillet. Common thickeners are fumed silica, wood flour and glass bubbles. Either approach works, both are best tooled with a nicely shaped fillet stick.