Luthier Tips du Jour Mailbag - CA Glue for Gap Filling

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In this episode, Robbie O’Brien answers a viewer’s question about how to fill hairline gaps where bindings meet the sides or back.

Video Transcription

[on-screen text reads: Luthier Tips du Jour Mailbag]

Mailbag question: Do gaps between the binding and the sides and back need to be fixed before applying the finish?

Robert O'Brien: Today's tips du jour mailbag question comes to us from Germany. "Dear Robert, I'm currently going through the finishing process on my guitar. I have noticed small gaps between the bindings and the sides and back. Do these need to be addressed before applying my finish? Manfred in Germany."

Manfred, in a word, yes, these need to be addressed. Anything you don't want showing up under the finish needs to be fixed before you apply the finish, because the finish will only magnify any imperfection that was not caught during the prepping stage. Now, it's not uncommon to have some small hairline gaps where the bindings meet the sides or where the bindings meet the back. Now, ideally you don't want these, but hey, it happens.

Now you can use a pour fill to fill these gaps, they'll be level under the finish, the finish will be nice and level. However, depending on the pour fill that you're using, that will show up under that finish. For example, if a clear pour fill is used to fill the gap, then you will see the gap under the finish although the finish will be level, let me show you a method that I use that will make the gap invisible under the finish.

Using CA glue and sanding to close hairline gaps

I like to look around the edges where the binding meets the back and the sides, and if there's any little gap like right here, I'm seeing just a little hairline gap right there, the finish will suck down in there and you won't get a level finish. The surface being extremely level and smooth is very important to a gloss finish like I'm going to be doing with the shellac.

So what I like to do is fill those. Now, you could let the pour fill do that, but I like to go ahead and do a preliminary filling. And what I'm going to do is just use CA glue [on-screen text reads: Hot Stuff brand CA glue available from LMI or Satellite City]. In this case, I'm using the thin viscosity, which is almost like water, and I'm just going to place a little bit along that area. Then I'm going to come in with an old piece of sandpaper and sand. And what that's going to do is the sawdust that's created grabs hold or mixes with the CA glue, gets down into the hole and fills the little gap. Make sure you get the rest off the surface so you don't have a little dark-colored area. Let me see if I can get a close up and show you how that works. So here's my gap, it's not very visible but there it is. See the little hairline gap right there? And that's what I'm going to try and fill. So I'm going to come in just a little bit of the thin viscosity CA glue.

[Robert pours a little CA glue along the micro crack then runs a razor blade over the glue]

It doesn't take very much, just put a little bit on there, if you want to even kind of help it flow down in there with a razor blade, you can. Okay, then I'm going to come in with an old piece of sandpaper, create some dust, and there is now no longer any hairline gap between the binding and the sides. So I'm going to do that all the way around the back, all the way around the sides.

Now around the top doesn't work very well, cedar and spruce, that trick does not work well on because the CA glue gets in there and stains the cedar or the spruce. So when you put your bindings in around the top of the back, you want no gaps, you want that to fit in there nice and tight. So I'm going to take a quick look around the guitar on the back and the sides and see if I can find any areas like that that need to be fixed.

So Manfred in Germany, I hope this information has been helpful, and make sure that you address any gaps before the finish goes on, and I think this method will work well for you. Happy finishing.

[on-screen text reads: More Luthier Tips and online courses available at Private and small group guitar building and finishing instruction available.]



Robbie O'Brien

Luthier and Instructor, Lutherie Academy