Bracing for a thin soundboard

[fretman314]

After sanding out the tool marks on the soundboard of my IV kit I have found it a bit thin in places (typically centering around the middle of the treble area). I want to reinforce it with some sort of bracing. This is my first build so I'm happy to compromise on any sound degradation. I have also not cut any form of soundhole yet and am not sure whether f holes or a single oval will compromise the top the least.

[fscotte]

How thin is thin?

[fretman314]

See my post "IV kit revisited". It has a profile pic on it.

[fscotte]

It appears your about .100" in the areas where the treble foot of the bridge would sit? If so, yes that's thin.

If you want to use that top, I wouldn't be too worried about sticking to traditional bracing methods. Its hard to recommend a specific bracing without having it in my hands to check for stiffness. But don't be too worried if you have to use 3 braces.

[multidon]

If it were me, I would think seriously about fashioning a patch for the thin area. The edges could feathered at the edges to blend right in with the rest of the top. If you did it cross grain, or perhaps 45 degrees to the grain, it would be very strong. You could think of it like you were cleating a crack, only pre-emptively. Reinforcing patches like that are common in violin restoration work.

I remember a thread a while back when someone took a Gibson Goldrush to a shop to have some work done that involved taking the back off. It had never been repaired before, and to both the owner's and the repair guy's surprise, they discovered such a patch on the top that must have been done at the shop! And this was a 6000 dollar mandolin!

[fretman314]

Thanks for the replies. I'd not considered a patch but it would be more difficult to ensure a perfect fit to the underside compared to shaping a brace.
I had originally intended a single oval sound hole with a X brace. Having looked at guitar bracing patterns it looks like I have a wide range of choices. I need to ensure with grain strength too as well as across grain to support the bridge.

[fscotte]

This one?

[multidon]

Not sure if that's the one I saw. I thought it was the back that was taken off, not the top. But I could be wrong. Wow! Is that ever sloppy! If I was doing that, I would feather the edges way more and use, ummm, maybe just a little less glue?

It is true that it would be a little more difficult to fit than an extra brace, but less impact on the sound, I think.

[buckhorn]

On one of my builds, I found a really thin spot on my figured maple back.. the spot was about where the fingerboard extension was located , about 1in around.. it was soft to the touch.. started with a little TiteBond and covered with saw dust.. did several layers the same way.. at first the wood swelled out, but when dried it disappeared.. a little touhup sanding proved it was level.. inside, I tried to feather out the glueing fix.. soft spot was gone and sound was never affected... Keith

[HoGo]

I remember that Goldrush, mess.
I did quite large reinforcement patch on old Schoffield mandolin with weak thin top. I added 1/16" thick layer to most of the top between the f holes. Instead of chalk fitting large block of wood like violin maker would do I did plaster cast for the top to sit in (I did some arch correction as welll in that case) and added 10x1" strips of split spruce one next to another (after the previous dried) using go bar deck. Then regradated the patc hand tapered it to nothing at edges. You can easily add such strip(s) to reinforce your weak spot. Here is pic fom the procedure:

[fretman314]

Hi Adrian. Sounds like the way to go. Good idea about making a mold. I've found a supplier who can supply a slab of bracing material to cut the strips from. As a matter of interest what glue did you use? I'm thinking of hot hide glue but am inexperienced in using it.
Steve