I would avoid using "lacquer with nitro celulose base" by all means (unless you have steel-string guitar in mind), same for shellac finishes. Leaving soundboard without any treatment is also not good. I would recommend using quick drying oil based finishes, such as Stand Oil (available in art shops) or, better still, Danish Oil (Rustins or Liberon brands), a couple of coats of which will do the job. This last finish dries fairly quickly but if your maker has a UV cabinet then it can speed up drying time quite considerably (my does it in 30 min!).
Shellac may have less dampening properties than oil - or at least it does according to Schleske and many other people. The amount of oil that is put on your typical Lute or Baroque guitar soundboard is extremely small. The last instrument that I made had the Danish oil finish (soundboard) that Alexander mentions. It's wipe on, wipe off. Certainly wipe off the second coat very soon after application, otherwise the finish will look very patchy. When it's thoroughly dry it appears as though no finish has been applied. Prior to the Danish oil I used Tru-oil, probably not that much difference between them. Maybe Tru-oil is slightly more like a resin based varnish.
From my experience it doesn't seem to hinder the response or tonal properties, at least in the amounts we are referring to. My last instrument didn't seem to think so either but I must admit to being a little worried about putting Danish oil directly on to a soundboard. Fear not. Alexander uses it. David van Edwards uses it. Now I use it. Just don't let it build.
I can't see any point for concern here. Quite the opposite in fact, because all oil based finishes (including True Oil) would be 'lighter' than those with resin as their main ingredient simply because the amount of oil that is introduced into the wood structure would drive out some amount of water that is naturally present there (linseed oil density is less than water). I think it was Joseph Michelman who virtually "soaked" soundboards of his violins in linseed oil and they turned out to be a few grams lighter after that.
You are right, Michael, that the actual amount of oil (and whatever other ingredients there are in the Danish Oil) is very small and in that respect whatever kind of oil-based finish is used the end result will be pretty much the same. The point here (well at least in my understanding of the idea of baroque guitar / lute soundboard finish) is to only seal the exposed pores of wood so that the dirt doesn't penetrate there. Over the years I tried all sorts of different finishes for my lute / vihuela etc soundboards (egg tempera, stand oil, tru oil, Danish oil, oil + resin based varnishes etc) and I couldn't really notice any diverse effect on the sound either. I must say I never had problems with the Danish Oil getting patchy, that's a real surprise for me! Again, I'm talking about very little amount of finish here and if there is a noticeable build-up of it on the surface I would buff it down with 0000 steel wood, followed by finer-grade sandpaper and / or burnishing paste, whatever is necessary.
As for the idea that shellac has less dampening properties than oil (whether it's oil on its own or in some sort of oil and resin based varnish) that may well be true but I'm not sure if it is all that relevant to the sort of finish we are discussing here. In fact, if Schleske and others who conduct such experiments were relying more on their ears (such as in 'blind test' for example) rather than electronic equipment the results (i.e. as judged by the actual perception of the sound) could be different. Oil based varnishes certainly appeal to me more than spirit based ones simply because of their superior visual properties, ease in application, sheer flexibility in colour modification (whether it's done by glazing or direct addition of colour mordant) and, not to mention, great historical tradition as is well testified in the De Mayerne Manuscript.
I've used Danish Oil from Rustin on all the instruments I've build (2 4c guitars, 1 vihuela de mano, 1 vihuela de arco and 1 fiddle) and am very satisfied with the result. As Alexander mentions the amount of oil actualy absorbed by the wood is small (most of it ends in the cloth used in wiping off the surplus I think).
On the first instruments I put 7 'layers' of oil resulting in a silky shine.
The last 2 instruments I've treated with egg-white prior to the finishing with oil.
(Main purpose to harden the surface of the wood). After this approx. 3 layers were enough to achieve approximately the same result as the 7 times treatement with the danish oil.
While applying the oil I got the impression that the older the oil is the longer it takes to dry/harden. (I don't have an UV cabinet).
Schleske uses a very scientific approach. It's a long time since I read his results but he tested various finishes that had been applied to strips of Spruce. Shellac added mass and resulted in what Sacconni referred to as ossification of the wood - adding a certain hardness or stiffness. He found that oil increased mass but also increased damping. Perhaps that's in direct contradiction to Michelman, or I'm not recalling 'facts' correctly. The oil and damping part I'm quite sure of. I've absolutely no basis to question Schleske's results but again the amount of oil used on our soundboards is extremely small. That may be the crucial bit.
The brands that Alex. mentioned (certainly Rustins) are a blend of oils and resins. The resin content is probably fairly low which would make it a 'long oil varnish'. It it is thinned with a solvent so that it is water like in consistency and easy to apply as a wipe on finish. Those brands may not be available in the US. but you can bet your last cent that there will be an equivalent.
I haven't used the Tried and True. E-mail Joe Robson and ask if his wiping varnish has a resin content. Joe also specialises in Violin type oil varnishes ie. brushed on.
The other alternative is to use Tru-oil. I've used it on soundboards but cut with approx. 30 - 40% turps. Whether Tru-oil contains some resin content seems to be something of a mystery but it certainly acts like it. At the very least the Linseed is highly polymerised.
For necks I use the wipe on danish oil. Backs/Sides is either wipe on (for a matte finish) or a short Violin type oil varnish for a gloss finish. Uncut Tru oil can be used on the Back/sides to also give a gloss finish but you will have to apply in excess of 10 coats to achieve that.
The 'trick' is to practice on off cuts first. One day I'll heed my own advice :-)
I doubt that the oil in these products are sun thickened. Most are probably heat treated to accelerate the polymerisation.
Oliver: use something on the Soundboard. Personally I wouldn't use nitro cellulose based products but many makers of modern Guitars do.
It was originally formulated for high traffic flooring so I figure it must offer good protection to guitar tops. There is a second product that goes on after giving the oil a few days curing time and that is a Hard Wax. So far excellent results.
Both "Tried and True" and "Naturhaus" products are enviromentally friendly which I like as there are enough poisons in my shop as it is.
Did you use the Tried and True Varnish Oil on just the soundboard or the whole thing?
I notice on their site http://www.triedandtruewoodfinish.com/ that they state the Varnish Oil builds to a semi-gloss, whereas their Danish Oil (which is 100% Linseed oil) creates a satin finish. I could envision using their Danish Oil for the soundboard and their Varnish Oil for the remainder... Any thoughts, anyone?
I agree with you that I like the fact that their products are environmentally friendly and safe -- no petroleum distillates or heavy-metal driers! I've used Tru-Oil in the past, but I'm fairly sure it is loaded with metal driers... Both Tru-Oil and Tried-and-True do contain polymerized oil -- part of the trick to getting it to dry so quickly.
I should note here for others that Tried and True does NOT contain any thinners or solvent -- it is just the polymerized oil (and/or resin).