Official Luthiers Forum!

Hi Folks, I am about to start cutting out an OM size top from Sinker Redwood. I have used an X-Acto to scribe the outside shape and plan on doing the same for the Rosette. I can already tell it is going to be slow going when cutting and ultimately routing. I have read previous posts that talk about sealing end grain with shellac but I wonder if I should use a 1lb cut applied before I cut out the top, then seal the end grain? I picked up some Zinsser Seal Coat and plan on cutting it to a 1lb cut. I have done a bit of repair work in the past but this will be my first acoustic guitar build and I am taking it slow and careful. I plan on using Adirondack spruce for the bracing and a maple bridge plate (both purchased when I bought the redwood). Currently the top thickness is 2.9mm. I don't have a thickness sander so the thicknessing will need to be done my hand. I know the subject of final thickness has been addressed in prior posts, but I have heard that most folks stay close to 2mm and I think Taylor uses that thickness Any advice is certainly welcome.

2 mm is somewhere under .080.
That would be thin for any top. That is more like side thickness. I think most will tell you that stiffness is more important than thickness, but at 2.9(.114”) I think you are getting on the thin side, especially for redwood.
Thats my 2 pesos worth.
B

_________________
My memory is so good, sometimes I remember things that never happened.

I’m with Bri. I haven’t used a lot of redwood but it’s always wound up a bit thicker than spruce...

2mm is almost certainly too thin.

Some of the better makers of classical guitars go down to 2 mm for their spruce guitar tops (sometimes a little thinner) but those guitars have much less tension and a smaller body size than an OM. I agree with Brian, at 2.9 mm you are probably as thin as you want to go.

I'd agree that you are probably close to done on the thickness of that top but it depends on the size of the guitar. I've built small parlor guitars with tops as thin as .075in. The two OM's I am building right now according to my deflection testing and the Gore Gillet acoustic testing has me down to .103 and .110. So that puts you in the ball park.

I have a fair bit of sinker redwood. I use dewaxed shellac. The wood sounds great. Treat it like spruce for thicknessing. It is typically quite stiff. It is also a bit brittle. It will not take strikes well. Rout with care - sharp bit, and do not take too much off. If you want to take extra precaution, you can always use a spindle sander to get close to pattern, then rout. Follow climb cut protocol and such.

What I’ve found with redwood is that it is very inconsistent piece to piece. I’d suggest building a deflection testing jig and doingvto measuring. There’s another thread going round here right now with some good info on that...

I/ve never used sinker , but Have some wide grain redwood abt 25 yrs old .I had a lot of problems with the top. Mine had a tendency to split at the most in appropriate times . Lots of CA., and as others said its soft , and needs vy sharp tools to get a nice clean cut. Take your time an practice your cuts on scrap first,

I built my archtop with old curly redwood, but it wasn't sinker. Being an archtop, and my first guitar, I have no idea what a flattop would thickness at; but IIRC (not a high probably!) it was 2.5- about 3.5 mm thick, a little thicker maybe right in the middle, with modestly sized parallel bracing. I used Z-poxy to finish, just a couple very light coats; and it doesn't feel the least bit prone to nicks or scratches. It's like a steel part being carburized and hardened. Really quite amazing, I was a little concerned about softness, but it is not a problem now.

_________________
Why be normal?

I'm building one right now with a curly redwood top. Gave it the Z-poxy treatment that was then sanded off. Sitting at .120" now before final sanding.

I will freely admit, I have no idea what to look for in terms of deflection. I don't have a set up for that but have seen various ways on YouTube. Chris Paulick's version seems the simplest, but again, I don't know exactly what I am looking for. I am slowly but surely getting the tools necessary for building. I either make them or buy them. I work in theatre scenic construction and my shop has a thickness planer (Delta 13"), but no jointer or thickness sander. We just don't need those more specific kinds of tools. The top I a.m working with was purchased from Lmii. I had them join and sand. At the time I didn't specify a thickness to sand to and just assumed they knew best. That was 4 years ago. I've been holding on to it till I had the time and tools to build. I will probably be thicknessing by hand.