Question for archtop guitar makers

[Bass.swimmer]

I was working on thinning a top, and I accidentally made the top too thin in a small patch (about 3 inches square). Is there any good fix for this? Or would it be ok?

It's actually a cello top (not a guitar), but I thought some techniques could transfer.

[Alan Carruth]

Too thin as compared to what standard? How much too thin? Where? What's the wood like? There are simply too many variables in play to be able to make any sort of determination based on your post.

[Bass.swimmer]

Sorry! The wood is Sitka spruce, quarter sawn, thinned to 6mm in the center sloping down to 4mm on the edge. The thin section is near the waist, about two inches from the f hole, and I think it gets down to about 2mm. The whole top is stiff, not flexing a whole lot. The wood has been air dried for over 20 years, so it shouldn't have too much water in it. This link should show some pictures I took of the thin section. If there's any more info you need, just let me know!

https://drive.google.com/folderview?...SkiAXkKKaRmj9R

[Bruce Sexauer]

Take the lesson and complete the guitar. Non-fatal error.

[Rudy4]

It looks even thinner on the other side above the f holes.

I'd recommend adding cloth reinforcement soaked with Original Titebond. The same type of material as is often used for side reinforcement strips. The cloth will conform to the irregular shape of the thinned area and help prevent it from developing cracks.

Gauze or course-woven cotton is often used by those of us that make f hole arch tops to strengthen the area of the f holes that can be prone to developing cracks from mechanical stress in those areas.

[Alan Carruth]

What Bruce said: not wonderful, but probably not fatal either.

A cloth patch would help, as Rudy says. I've been using cotton-polyester bias selvege tape for reinforcements for decades. It lies down flat on curved surfaces because of the bias cut. When you get it from the fabric store it will be folded over, and you have to iron it out flat. I did some testing a long time back and found that I got a stronger bond using hot hide glue than I did with Titebond. The hide glue is also easier to use. That sort of glued on patch can come loose after a while because air breaks down the glue. To guard against that I've been shellacing over the tapes: we'll find out in 75 years or so how well it worked.

[MC5C]

I use a single layer of fiberglass cloth, fine mesh weave, with West System marine epoxy to reinforce around F-holes, kind of the the modern equivalent of the glue and linen cloth trick. That would reinforce your thin patch very well. I use an old trick of saturating the cloth onto the wood with epoxy, then absorbing as much as can be taken away with paper towel, leaving only a minimum of wet epoxy to set up and bind things together.

[RoyBoy]

2mm Yikes! That's violin thickness for the area of minimum thickness. I'll bow to the more experienced voices who've offered suggestions here, but I'd be tempted to laminate a few more mm of spruce with hot hide glue. The fabric/glue suggestions are wise because that's an area that is prone to cracks.

In learning to carve mandolins, I overzealously carved a couple of instrument tops too thin. Desperate to not waste good wood, I glued the instruments up in the white without binding. They sound terrible. I desperation I resawed some more of the spruce and laminated in thin layers with hot hide glue to the inside of the tops to bring them back to up to spec. Reglued the instruments to try again. They still sounded terrible to my ear. I ordered more spruce.
BTW, those laminated spruce tops burned beautifully in the campfire.

But…a cello top is a LOT of wood to waste, so go with the remediations recommended and hop for the best.

[Bass.swimmer]

Thanks for all the advice! Yeah lesson learned for next time. I think I might add patches to the big thin spot, and lay in curls with hide glue in the very thin spots on the left side.