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I have never finished the inside of my builds, but I have this very figured cherry OM I just started and wondered if I shuould shellac the inside. I intend to put in a sound port and you will be able to easily look inside. In this picture I wiped on some naptha. Any thoughts about shellacing the inside? (except for the top...)

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Some people like to shellac the inside to slow down moisture exchange from the air. It seems to help, a little: there's no way to stop it entirely.

Shellac on the inside is a problem down the road when repairs are needed. Glue doesn't stick to it as well as it should, so if a cleat is needed the repair person has to scrape the shellac off.

With pretty wood like that I think folks will be too busy looking at the outside to pay much attention to the inside...

You can sand the inside to 240 or 320 before you put it together. A touch up with the sandpaper to the sides and back before you glue the top on will leave a nice looking finish even without the shellac.

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Steve Smith
"Music is what feelings sound like"

A light coat of shellac on the inside might bring out the curl without creating too big a problem for future repairs - something that can be removed with the light sanding that should be done to remove dust and dirt before cleating.

I consider it a waste of time and shellac and it does degrade to a small extent serviceability as mentioned already above. We can get around the serviceability issue but for what value to anyone?

I'll add that we do see a very few people's instruments who did this and I don't even think it looks nice. There is no sheen so it simply looks a bit wet.

One more thing. Many steel strings these days will get pick-ups. Putting shellac where a pick-up has to be installed with adhesive tape such as a Baggs Element or Anthem also means that the finish has to be removed even for a pick-up install. I would never put finish on the underside of the top and then you have K&K transducers that will need to be glued on there too....

With this said I see finish on the inside of the box as possibly, depending on the player a detriment to how they intend to use and perhaps gig the instrument.

with hesh adds cost with no real benefit

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I don’t shellac the inside of instruments I build but some prominent builders are doing this. I would expect to see more of it going forward.


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With more sound ports giving a birdseye view of guitar internals, it's natural to make the construction clean and neat and shellac would make it even prettier. Looking in the port is a trip into the guitar underworld and I love the look. Ports go in all my gits unless requested to not have one.

Shellac is also one of the better finishes at slowing the transfer of water vapor into the wood so it might be of some benefit to the shock of taking a guitar outside on a 95% day. But you really need 3 coats to really slow the that process.

Even thou I consider the inside as pretty as the outside of the box and maybe more interesting, I just try to keep my work clean and won't shellac the inside due to the repair reasons already mentioned. It's hard enough to make internal repairs without a finish getting in the way too.

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Brian R, Wood Mechanic
N8ZED

More so on the classical side than the steel string side. There have always been builders, some prominent that shellac the inside it's not new by any means.

Slowing the absorption of excess moisture or perhaps we should say shielding the instrument from RH swings in both direction is pretty minimal as you said with shellac perhaps only several hours tops so I really don't see any benefit there either.

I feel there is no real benefit from a single layer of shellac, but not much harm either if you want it to look pretty inside.

After reading the results of the RATE of moisture transfer testing done by the US Government on hundreds of various wood coatings/finishes, I started using shellac to finish the inside of my builds. The government compared the rate of transfer with the coating to the rate of bare unfinished wood (ala, the inside of a typical guitar).

The rate is several times faster for unfinished wood compared to shellac coated (shellac on one side and whatever finish on the wood's other side). How does this help? Have you ever heard a professional player complain that his guitar was playing great on one coast and then he traveled to the opposite coast and the guitar had new buzzes or other issues after a couple of days? Typically, this is due to weather change, especially humidity. It takes very little time for a guitar with raw wood to give up or absorb moisture. It can, according to government tests, change quickly. This is especially true of thin veneers with large surface areas.

Consider two guitars, one finished inside and outside, and one outside only. They are placed together in a room with constant weather for a month. Both will equalize during that long period regarding moisture content. Therefore, in a constant or slowly changing weather environment, there is no real advantage to finishing the inside. But if a new weather front rolls in that significantly changes the weather for a few days, the unfinished guitar will move a significant amount with the change while the finished guitar will not. Or as stated above, if the two guitars travel to a new location, say from the warm winter south to the dry cold winter north, the rate of movement between the two guitars will be significantly different.

My understanding is that unfinished wood would equalize in about a week compared to a month for shellac. So SLOW RATE OF CHANGE is the advantage for finishing the inside with shellac. With slow changing weather or constant location, there is not real advantage. (I had a professional tell me that a guitar I made him "never goes out of tune".)

The attractiveness of a finished inside is just a small is a side benefit.

Here's part of the moisture exclusion table from USDA, Wood Handbook. It is worth considering when applying finishes anyway and useful info to be aware of.

Whether we finish the inside or not is like so many decisions in instrument making - a compromise between strength, tone, appearance and repair-ability. Up to us to draw those lines where we can live with them.

As I've said earlier, I'd like to put 3 coats of shellac inside, but the repair aspect stops me. In my furniture building days, we always did the underside of solid wood tables or inside of cabinets to provide some level of RH shock load protection on large the solid wood panels. Especially in the midwest where RH can swing a lot in a couple days.

As much as we'd like, instruments just can't be kept at a constant RH unless they never leave the house. True story.

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Brian R, Wood Mechanic
N8ZED

Great stuff Brian I was comparing shellac to nitro and that got me wondering if they were measuring dewaxed shellac or the standard variety, I suspect a difference in hygroscopic action there too.

Just to set the stage in the over a thousand guitars we repaired last year, 2023 I can't recall one with shellac inside.