#16
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#17
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Breathing is an essential aspect of the guitar box. If the woods were sealed by the finish then no acclimatization could occur after the woods were sealed. French polish wood allow the lumber to breathe better than nitrocellulose, whereas polyurethane is a complete hermetic enclosure allowing nothing in nor out. The finish on the guitar is simply there to prevent soiling into the grain. An unfinished sound box would be the best way to go, relative to acclimatization as well as sonically. Think of putting plastic wrap over the top of a speaker. It's no different than the finish over top the wood grain.
The visual component to musical instruments is so strong in people that beauty alone is their initial, and usually uninformed draw to them. Hence the thick, (poly) deep looking finishes, shot-gunned MOP and a whole host of aesthetics luthiers put into their market feedback compliance products. You want that? Okay... |
#18
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It seems that many are overlooking the fundamental purpose of treating wood. It's to enhance the appearance and/or to protect it.
If you're not going to see it, and it doesn't need protected; finishing it is a waste of time. Do any of those tables with finished undersides the customers care so much about also have wood chairs? Flip one over. Dollars to donuts the underside of the seat isn't finished. |
#19
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Applying a finish to the underside of the soundboard will only serve to further dampen its vibration.
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#20
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#21
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I seal the inside of my resophonics. Resophonics are basically speaker boxes, so I doubt there's much sonic difference between sealed and raw. In a few years I'll let everyone know if the sealed wood has been damaged in a way directly related to the seal.
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#22
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Why? Painters do it all the time. What's the difference with spraying finish vs. spraying paint that would make it hard to tape off where you don't want finish to go?
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#23
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We are not building furniture so the principles dont apply as seen in a million Martin guitars. Its ok to finish the back/sides but not the top as this adds weight and therefore suppresses sound.
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#24
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Me too. That’s my point.
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#25
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Table tops are attached to the skirt at the edges, and that helps to hold them flat. Piano lids are free floating, and could well warp if they were not finished on both surfaces. I talked with a fellow once who bought wood for Steinway. He was always on the lookout for the widest perfectly quartered poplar boards he could find.
There is no such thing as a finish that absolutely seals out moisture. The Navy tried to find one that they could use on their wooden 'P' (personnel) and 'U' (utility) boats, which are used to shuttle people and supplies back and forth between ships and 'the beach' in ports that don't allow for docking. Even 1/4" of epoxy wouldn't do it. That's when they started to switch over to boats made from fiberglass trees. As usual , the term 'damping' is being used rather loosely in this thread. Although it's often taken as referring to anything that reduces the amplitude of vibration, that's not strictly correct. In acoustics 'damping' refers to things that dissipate energy; analogous to resistance in an electrical circuit. Continuing the electrical analogy, increasing the inductance in an L-R-C oscillator circuit would lower the resonant frequency, in the same way as adding mass on the top of a guitar by putting on more finish can drop the 'main top' pitch, but the actual loss within the system may not be any different. Adding in a resistor could leave the resonant pitch unchanged, but would add damping that would cut down the selectivity of the circuit if it's being used as a receiver. In the case of the finish on the top, note that adding more may drop the top pitch. A lot depends on whether the finish adds more stiffness or mass in proportion. Most film forming finishes add at least some stiffness across the grain, and may even add more than their share along the grain. They all add mass, though. The added stiffness may be enough to preserve, or even raise, the resonant pitch relative to that of the unfinished top, but for a given signal strength the amplitude will probably be less. That's not evidence of 'damping' though. One usual measure of damping the 'Q' value of the system, which is a measure of the proportion of energy dissipated by the system per cycle of vibration at it's resonant pitch. There are various ways to measure this; one is to simply count the number of cycles it takes for the amplitude to reach some set proportion of the starting amplitude. In the case of a guitar top with more finish, and thus a lower resonant pitch, this would take longer, since there are fewer cycles per second. This may not be a straightforward thing to do on a real guitar, since there are other parts, like the back and the air inside, that can steal energy from the top at certain frequencies. In terms of this measurement the sound that the guitar produces is a 'loss', but we want that (within reason)! Measurements that have been made on wood strips finished with various materials give some idea of what to expect in terms of damping and stiffness from different ones. Basically, any finish containing a drying oil adds some damping. Plain oil finishes add the most, and also can add significant mass. They tend to soak deeply into the wood, for one thing, and many don't form a true protective film once they cure, so you need to keep adding more to get the benefit. Oil-resin varnishes of the sort that we use, termed 'rubbing' varnishes, tend to add little damping, and for decent films. Shellac adds some stiffness, and actually has lower damping than most woods do , at least across the grain, so adding shellac can actually give effectively lower losses for a given stiffness. Nitrocellulose lacquer is even better, at least when it's new. Nitro breaks down over time, of course, while shellac just gets better if it's not worn away or degraded by sweat. Like all wood, guitar tops suffer 'shrinkage hysteresis' as the hemicellulose degrades over decades. Since hemicellulose is the part of wood that absorbs moisture the wood takes up less and less over time, and gets a little smaller across the grain as a result. Commonly this leads to cracks over time. In order to repair them, you often need to glue a 'cleat' on the inside of the top. This is made more difficult by the presence of a finish. Finishing the inside may help slightly to mitigate somewhat the problems caused by sudden short-term drops in humidity, but won't keep the top from cracking over the long term. It might make the luthier who built the guitar look good, but eventually, when it ends up on some repairman's bench, he's going to get a cussing out. The bottom line, then, is that putting a finish on the inside of the top is not likely to confer much benefit, will certainly have drawbacks, takes time, and costs money. That's why you don't see it done very often. |
#26
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Well I'm glad I asked the question as there have been some great replies, thank you all who contributed.
I mentioned Maccaferri in my original post and I must admit that all the Macs I've ever played (and it's a few, I've been lucky) have had cracks in the top repaired at some time. Nick |
#27
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Unfortunately there’s a lot of Mis-information floating around out there. Polyurethane does not seal out all moisture transfer, nor does any other commercial wood finish. All finishes do is slow moisture transfer. Also as mentioned wood does not “breath”, nor does it need to exchange moisture necessarily. In fact the ideal state for a guitar or a piece of furniture would be in a climate that never changes, meaning the wood neither acquires or releases moisture. As most of us don’t live in such environments our wooden objects do in fact gain and lose moisture with environmental changes.
Now as far as using narrow boards to glue up table tops, the two main reasons for doing this are for economical and/or lack of understanding. A 4” wide board does not move less in proportion to its size than an 8”. In other words if you take an 8” board and cut it into 2 - 4” boards, then glue them back together the same way it’s going to move just as much as before it was cut. So manufacturers “cheat” by flipping the boards. This minimizes the affect of cupping by alternating it up/down, up/down. I’ve spoken with many guys over the years who utilize this method without understanding it. As for finishing the bottoms of tabletops I’ve seen it done both ways. My opinion is that for the most part it’s done for financial reasons, cheaper not to finish the bottom. I’ve done a fair number of solid wood counters over the years and always finish the bottoms. Never had one split, but as a woodworker I probably pay much more attention to the quality and condition of the individual boards I use than most bigger manufacturers. Guitars seem to attract a lot of interesting mythology. As a woodworker I think many commonly accepted ideas are more salesmanship than anything else, from tonewoods to finishes. In regards to the original question I don’t have an answer, but my guess would be a combination of tradition and cost. The finish on a guitar is really about protecting the wood, (which is extremely soft in many cases), from everyday dirt and scratches. Anyone who has handled a raw cedar or spruce top understands so much as looking at it funny will scratch it, so the finish protects it. Whereas the interior does not need protection. No need so no reason for additional cost😉 Good luck, Jeff |
#28
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Expansion and contraction, which can result in cracks or split joints happens when you don't account for it. Using a pinned breadboard end on a table is an example of how to mitigate the problem. |
#29
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I want to try and make this obvious statement without sounding condescending: There are lots of types of wood and lots of types or products made from wood and depending on what you're using and what you're making, the processes are often going to be different. A guitar top and a tabletop share almost nothing in common other than they can be made of wood - the species, the drying standards, thickness, joining methods, etc....totally different things so you should expect that they are treated differently in the finishing steps, as well. I understand the OP's curiosity and question, so completely reasonable to inquire.
I'll just say that in my opinion, finishing the underside would provide almost no advantages to the instrument and a cost/benefit analysis would 99.9% benefit leaving it raw. |
#30
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As usual, I am quite impressed by the well reasoned background thoughts that Alan brings to his answers. |