Acoustic guitar finish

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Matthew Atkinson
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Joined: Wed Feb 01, 2012 1:19 pm
Location: MA

Acoustic guitar finish

Post by Matthew Atkinson »

I would like to fully assemble my current project and then hand apply a finish. Is this practical? If so, what finish is recommended? I don't want high gloss.
-Matt

Steve Senseney
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Re: Acoustic guitar finish

Post by Steve Senseney »

French polish (shellac) would be a good choice.

The standard learning for this is the Milburn tutorial.

http://milburnguitars.com/frenchpolish.html

There are a lot of people on the mimf who do the French polishing and would be able to help if you have questions or problems.

Gerry Gruber
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Re: Acoustic guitar finish

Post by Gerry Gruber »

I would like to fully assemble my current project and then hand apply a finish.
Just to be clear, for steel-stringed guitars, finish is applied before the neck is attached to the body, and before the bridge is applied. For classicals, the neck and bridge are already attached. If by "hand apply" you mean to not use spray equipment, as Steve has suggested, you can go the french polish route. Or, as I did for my first few guitars, you could brush on nitro lacquer. To do so, you need to ensure the lacquer has sufficient retarder in it so that it doesn't dry too quickly. Some lacquer is labeled as "brushing" lacquer. I don't know whether you can brush on the "water-based" lacquers, but I suspect it is the same as with nitro. If you do go with nitro, make sure to use a proper fitted and rated mask.

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Charlie Schultz
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Re: Acoustic guitar finish

Post by Charlie Schultz »

Shellac or French polishing is pretty forgiving and not too smelly. It does typically give a high gloss, but (and for any finish), a very fine (1000+) sandpaper or rubbing compound can get you a matte finish.

John Hamlett
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Re: Acoustic guitar finish

Post by John Hamlett »

Another option is Truoil. I've seen and gotten excellent results with it, anywhere from matte to high gloss.

Matthew Atkinson
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Location: MA

Re: Acoustic guitar finish

Post by Matthew Atkinson »

Thanks all. Can Truoil be used on the top? Is that the same as Tung oil? I sprayed my first 3 guitars in a booth. I no longer have access to that. So full assembly before finish is not recommended? I will use the same procedure as before, masking off the bridge area and under the fb extension and finishing the neck separately.
-Matt

Bill Hicklin
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Re: Acoustic guitar finish

Post by Bill Hicklin »

Can Truoil be used on the top? Is that the same as Tung oil?
Yes, and No. Tung oil is very different and (according to everything I've heard) soaks deeply into wood and smothers the sound.

Tru-Oil is a proprietary polymerized oil originally marketed as a finish for fine gunstocks. LMII sells it, and I believe Stew-Mac as well. I've never used it on an instrument, but I have used it on other things and been pretty happy with it, although to get a high gloss it takes a LOT of coats. I like the fact that you can achieve as much or as little shine as you like, just by stopping when you're satisfied.

According to the LMII blurb, some builders finish their tops with something else, like French polish, and use Tru-oil for the back, sides and neck.

Alan Carruth
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Re: Acoustic guitar finish

Post by Alan Carruth »

French polish is a gloss finish. Applying it after the guitar is assembled is something of a pain (although that's what the Spanish makers have been doing forever); it's hard to get into the corners at the base of the neck, the end of the fingerboard, and around the bridge.

I'd get an egshell or matt varnish and brush it on. I don't know if the Behlen's 'Rockhard' varnish comes in eggshell, but I would not be surprised if it does. I thin it out about 25% with their proprietary reducer, and about 5% acetone, to a very watery consistency. This goes on pretty smoothly with a soft 1" brush. If you're careful, with an eggshell varnish you could level the fourth coat or so with 320 or 400 free cut dry sandpaper, and then just brush on another coat and be done. The Behlen's dries fast unless there's some oil in the wood (I've had Macassar ebony that required a week for the first coat to dry), usually four hours, so you could be done pretty quickly. DON'T recoat until the first coat is dry!

I've used Tru-Oil for necks once or twice. It's nice, but I think you'd ned to renew it every few months.

I've heard good things about wipe on polyurethanes. I used some once, 'way back, when one of the manufacturers sent a couple of cases to my shop buddy to try out. The plastic quart bottles had a label that folded out to about six pages of fine print. The first two or so detailed the hazards you ran by having the stuff in the shop, and then they got into the problems caused by actually opening the bottle. We opened one, and tried it out: it was, indeed, nasty, but made a decent finish. The rest petrified in the bottles over the next couple of months. I assume the new stuff is not so problematic...

Alan Carruth / Luthier

John Hamlett
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Re: Acoustic guitar finish

Post by John Hamlett »

A few things;
I've added flattener to Behlens Rockhard for a satin/matte/eggshell finish with good results (more flattener, less gloss; when you mix it yourself it's your choice). I simply used the silica powder that goes into the mixed flatteners that paint stores sell because I couldn't find a compatible flattener otherwise.
Truoil does not soak deeply into the wood if you don't want it to. Rub it on, rub it off immediately, let it dry, it seals the wood against subsequent coats penetrating deeply. Once fully cured it seems to be very durable on a neck, but it can be re-coated at any time.
Most of my experience with Truoil is applied over Behlen's Rockhard varnish. I can get anything from a mild gloss to a high gloss that way with no witness lines, and it all can be done by hand. I spray the Behlens and rub the TO, but that's because I have spray equipment and I'm much better with a spray gun than I am with a brush. I can keep the finish thickness very thin with that method. I reduce the varnish with lacquer thinner for spraying, but I'd use the proprietary reducer or mineral spirits for brushing (I've used both and can't tell the difference).

Alan Carruth
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Re: Acoustic guitar finish

Post by Alan Carruth »

I imagine there are several things you could use successfully to thin out Rockhard varnish. One thing you _cannot_ use is paint store thinner/ mineral spirits. It thins it out, alright, but forms a white precipitate at the bottom of the can, and the varnish stops drying properly.

Alan Carruth/ Luthier

Matthew Atkinson
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Re: Acoustic guitar finish

Post by Matthew Atkinson »

Thanks, everyone. I will try the Behlen's, I think. Can I steel wool it after it's completely dry?
-Matt

Matthew Atkinson
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Re: Acoustic guitar finish

Post by Matthew Atkinson »

Also- do I need a sealer coat under the Behlen's like shellac or sanding sealer?
-Matt

Alan Carruth
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Re: Acoustic guitar finish

Post by Alan Carruth »

One of the benenfits of using an oil/resin varnish is that it has a refractive index that's pretty close to that of the wood, so you get a nice 'light'. Any seal coat that you use is likely to have a different refractive index, and will give a 'veiled' look. It's particularly noticable when you end up sanding through, or sand the sealer partway back before putting on the varnish, and it looks splotchy. When I use a shellac and pumice fill I take pains to get all of the shellac off the surface before starting in on the varnish for this reason. The stuff in the pores is no problem.

Alan Carruth / Luthier

Matthew Atkinson
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Re: Acoustic guitar finish

Post by Matthew Atkinson »

Thanks, Alan. Can a pore filler be used under it? Can it be used quite thin as a sealer and then filled in between?
-Matt

John Hamlett
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Re: Acoustic guitar finish

Post by John Hamlett »

I usually seal with a thin coating of blonde shellac before using the Behlens, and it looks fine to me; I see no 'veiled' look whether on natural wood or dyed wood. The varnish has plenty of adhesion, however, so a sealer coat is not necessary. I seal, mostly, to "lock" my dyes in place before cleaning bindings.
Pore fillers can be used under it, but I certainly can't say that it is definitely compatible with every pore filler, so "test on scrap".

Alan Carruth
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Re: Acoustic guitar finish

Post by Alan Carruth »

I've used a number of different pore fillers with 'rockhard' varnish, and it seems compatable with all of them, so long as you sand back to the wood.

I'll note that we often talk about 'varnish' as if it there wwere no differences among the different brands or types. In fact, 'varnish' is apretty encompassing term for a variety of reactive finishes: things that cure by polymerization rather than the evaporation of a solvent. My experience over the years is that every varnish is a bit different, and there's a learning curve associated with each one. I've gotten to where I an get decent results with a couple of different brands of varnish, but YMMV.

Alan Carruth / Luthier

Erik Hokanson
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Re: Acoustic guitar finish

Post by Erik Hokanson »

I recently finished an instrument with several coats of shellac applied with paper towels then sand with 600 grit with an eraser backer and some slightly soapy water to lubricate, then when it dried for a few days I applied several coats of tru oil. The last coat I used the finest steel wool (which apparently is a no no, but it worked) and scrubbed lightly, and wiped ALL the dust off and layer on a last coat of tru oil. I have a low consistent gloss except for the spots that were not completely flat (red cedar top).

Matthew Atkinson
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Re: Acoustic guitar finish

Post by Matthew Atkinson »

I usually seal with a thin coating of blonde shellac before using the Behlens
I have put on 3 coats of shellac right out of the can, sanding in between. Trying to fill the grain a bit. Now I'm worried about putting truoil or Behlen's rockhard over it. I think I meant to follow John's advice and seal with it but for some reason thought I should use multiple coats to fill the grain (mahogany back and sides). Should I sand it all the way back before applying oil or varnish?
-Matt

Bill Hicklin
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Re: Acoustic guitar finish

Post by Bill Hicklin »

You can't really apply oil over a sealed surface; the whole point to an oil finish is that it's "in" the wood rather than on top of it.

Shellac however is just fine as an undercoat for varnish.

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Greg Robinson
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Re: Acoustic guitar finish

Post by Greg Robinson »

Well, that depends on the oil. Some oil finishes are "drying" and will polymerize without needing to soak into the wood, although it may take longer to do so if the wood is sealed. You would not want to use a non-dryig oil on a sealed surface though, but they do not really provide much protection anyway.
MIMForum staff member - Melbourne, Australia

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