French Polishing Newbie Mistake: Is there hope?

Construction and repair of Classical Guitar and related instruments
Pinewhisp
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French Polishing Newbie Mistake: Is there hope?

Post by Pinewhisp » Tue Aug 22, 2017 1:57 am

Hello all,

I am a hobbyist guitar builder who's been cruising along having fun on a low pressure, build-it-in-my-spare time classic guitar for the last three years. You folks have offered great advice in the past regarding hide gluing a fingerboard (that went very well, thanks to you guys). I've got a Swiss spruce top and eastern walnut back and sides, and although I've never done it before, I bone-headedly decided to finish the guitar by the more challenging French Polishing method to minimize tonal impact by the finish.
Well, apparently I have been having too much fun: I've committed what I suspect is a cardinal French Polishing sin on my beautiful walnut back and sides: After two initial sealer shellac coats, during pumice filling I apparently applied mineral oil prematurely—before a sufficient shellac sealer layer was adequately deposited on all walnut surfaces. The end result is some large dull-sheen splotches that refuse to take a shine on subsequent shellac body coats compared to the adequately sealed high-sheen surrounding areas (evaluated after oil-sanding with 600 grit and mineral oil).
I've subsequently rubbed on four body coats (just shellac and oil) and one more pumice-filler coat, but none of these has been capable of absorbing into the oil-contaminated splotches.
I've decide to place my tail between my legs, admit abject failure, remove the finish on the walnut, hopefully clean the oil from the contaminated walnut with a solvent like Naptha, and French Polish the walnut all over again—from scratch.
I'm wondering whether any of you expert-type luthiers can offer any advice to a newbie on the best way to strip off a failed French Polish finish. Paint remover? Sanding alone? Denatured alcohol?
Thanks in advance for any advice.

Kevin Ryan
Montpelier VT

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Waddy Thomson
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Waddy Thomson » Tue Aug 22, 2017 2:19 am

Alcohol will take the finish off if it's pretty fresh. I would try some spiriting off sessions to see if I could get it to even out and to remove some of the excessive oil. Spiriting off is just alcohol on the pad and glide on, glide off strokes in a straight line with the grain. Give it some time to rest and dry between sessions. May not work but worth a try. After a few sessions like that, let it rest again and try a few more sessions of polishing. Remember that a dry pad is better than a wet pad. Make sure to blot off most of the wetness on a paper towel or a piece of paper till you just get a light imprint of moisture. Then go to the guitar. Also, only tiny bits of oil. Less than a drop on the pad and well tamped in, so it blends with the shellac and alcohol and isn't just oil sitting on top.
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Alan Carruth
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Alan Carruth » Tue Aug 22, 2017 5:21 pm

What Waddy said. FP is in some respects the most forgiving of finishes; there are very few things you can do that will permanently mess it up. One FP expert I talked with told me that whatever the problem, the solution is more alcohol. He didn't say whether it went on the finish or into your coffee....

I note that the French furniture maker's method of FP actually starts by flooding the wood with lined oil. This is allowed to sit for a little while, and then wiped back to pick up as much as possible. I've tried it (not on a guitar) and it does work. A small amount of oil stays in the wood, and most of the rest floats to the top to act as a lubricant. Some of it mixes in with the shellac, too, and, being a hardening oil, becomes part of the finish. In theory this would impart some added toughness and possibly chemical resistance. There is some evidence that the old Italian violin makers used this sort of technique in their 'ground coat', although they may have used some resin other than shellac. This is pretty controversial, of course: in one case somebody on line even objected to the use of the term 'ground coat'. The oil in the wood does enhance the finish visually, however drying oils like linseed and walnut oil also add damping, which is probably a good reason to avoid getting them into the wood on guitars. Violins don't suffer as much from added damping, and may even benefit.

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Steve O
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Steve O » Tue Aug 22, 2017 5:58 pm

My problems with French polishing have always been due to several factors:

1. Too much oil. Use a minimal amount of oil. Too much oil has caused soft finish in some of my past projects.

2. Not enough oil. Use just enough oil to prevent sticking. If your pad does stick and raises a rough spot, then a quick sand fixes the problem.

3. Polishing too long or too often. This is where I always screw up. A guitar is pretty small and it is easy to go over an area too many times or repeat a pass over an area too soon. If you detect stickiness, the tendency is to add more oil and keep on building, whereas you have probably overworked the surface should have taken a break to let the finish dry.

If you overwork the surface, the previous coat is still sticky and you will pull the finish and get dull spots. These are easily repaired by waiting for the finish to dry, then sand the area with 600 grit to remove roughness, and then continue polishing.

When polishing a guitar back, I would probably do a ~30 second session, wait 15 minutes, then repeat 3 or 4 times before leaving the guitar overnight to cure.

4. Too much pumice when grain filling. I had one tabletop that you can see the rottenstone under the finish.

5. When spraying shellac I've had problems by applying too many coats too quickly (or perhaps coats that were too thick). The intermediate layers were not fully dry (even though they felt dry) before applying the next coat. The resulting finish looked great, but it imprinted very easily and too eons to fully cure.

6. I've bought two bad batches of shellac flakes. I had some difficulty fully dissolving the flakes and the resulting finish was soft and imprinted easily. If the flakes do not completely and rapidly dissolve, or if there is any gummy stuff settling to the bottom of the jar then I throw out the shellac.

7. I'm no expert in French Polishing. My early work used mineral oil as the lubricant. Later work used Linseed oil. I was trying to figure out why some of my shellac finishes were so soft. One person's theory was that I used too much mineral oil, and also that the mineral oil never hardens. Linseed oil (or polymerized tung oil) eventually harden, eliminating one potential issue. ???

I am surprised that you cannot sand back and then start building up the finish again. I've always been able to fix dull spots by doing this. When sanding shellac, I like to wet-sand with low-odour mineral spirits; I would not wet sand with oil. The mineral spirits readily evaporate whereas the oil will remain on the surface, and too much oil has caused soft finishes for me in the past. I use low-odour mineral spirits because they don't stink as much as regular paint thinner, but both work well.

John higgon
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Re: French Polishing Newbie Mistake: Is there hope?

Post by John higgon » Tue Aug 22, 2017 7:05 pm

There is a very useful DVD - "The Guitar Maker's Guide To French Polishing" - by James Lister of this forum. It really does take you through the process step by step, with advice on common pitfalls and how to avoid them. (I hope I'm not breaking any forum rules by pointing you in this direction.). Next to one-to-one lessons, a DVD is the best medium for learning as you can see exactly what's happening, which you can't do when reading a book. I got a reasonable finish following James' instructions on my first attempt. I'm sure practice and experience also play a large part.

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Re: French Polishing Newbie Mistake: Is there hope?

Post by MessyTendon » Wed Aug 23, 2017 4:48 am

There is hope...Yeah try to get the mineral spirits out with some solvent. Keep working out out. Just lightly soak and keep wiping.

Denatured alcohol is nasty, but evaporates quick and it's cheap and easy to find. If only 4 coats in, you can easily remove it...maybe do some light sanding.

If the paper doesn't load up with oil, you'll know you are more or less free of any residual oil...400 grit or higher, maybe go 600 grit...just be patient when sanding.

The nice thing about a french polish is you don't have to make it look like a plastic piece of ice skating rink material...that is so annoyingly shiny.

There should be only drops of oil, but it needs to be a drying oil...almond oil, avacado oil, flax oil works. Only a few little droplets just to get the bad going.

If you over oil, that does create spots. It's the shellac you want moving, the oil is just to help the pad glide a bit. I do believe once the shellac starts curing, it's a pretty durable finish.

Plenty of time to start over...I don't think the word coat is a good analogy for shellac. Think of a shellac finish as you are trying to spill a glass over water, and you want one uniform layer of liquid to be a certain thickness.

It's nearly impossible to knock of a glass and get a perfect uniform coat...So when you use a bit of solvent in the shellac, it is moving more of that shellac around.

Once you have polished enough, it's as if you spilled that glass of water perfectly and created a great shine. If it was as easy as putting on a few coats, people wouldn't proclaim french polish to be a complicated art, but it's not...take your time and try to spill the glass, little by little.

Pinewhisp
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Pinewhisp » Thu Aug 24, 2017 9:57 am

I want to thank everyone for your abundant advice on how to extricate myself from this problem. Rubbing with alcohol is not making a dent in the islands of oil-in-the-walnut dullness that I want to remove, however. So, I will proceed to sanding the finish off and applying a solvent on the oil-contaminated areas before I begin the process anew. Based on your advice I had definitely been applying gobs way to much oil way too early in the process (think 747 when a paper airplane was all that was necessary, and think touching down before the runway has begun).

I've been following Ron Hernandez's video on French Polishing and on review I notice that he charges his pads with equal parts of 2 lb cut shellac and alcohol. Am I wrong to charge my pad with 1 lb cut shellac that has already been prepared before charging?

Thanks again to everyone for your time.

vesa
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Re: French Polishing Newbie Mistake: Is there hope?

Post by vesa » Thu Aug 24, 2017 10:27 am

Pinewhisp wrote: Am I wrong to charge my pad with 1 lb cut shellac that has already been prepared before charging?
Many different opinions about that.
When bodying I charge my muneca with 8 drops 2 lb cut,
6 drops alcohol and 1/4 - none drops olive oil.
When glazing 3 drops 1 lb cut, 3 drops alcohol and almost without oil.
Amount of the mix depends of course on the size of the muneca,
but I think the relations I use are very common.
Vesa Kuokkanen

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Michael.N.
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Michael.N. » Thu Aug 24, 2017 12:21 pm

Why bother. Why not just dilute the shellac to the cut that you want to use. Never really understood the 6 drops of shellac, 6 drops of alcohol thing.
You need to get rid of the oil that's wicked into the wood. That might be more difficult than you think. If you can't it's always going to give you a halo because oil tends to darken wood more than straight shellac. It also pops the grain better than shellac. The other alternative is to coat the wood with a drying oil and hope that it evens things out. As Al pointed out it's a traditional method of French polishing. I think it's fine for the back/sides. I use that technique myself because it helps to give a lively look to the grain (that oil thing).
I tend to use anhydrous isopropanol as the solvent. I don't really notice any difference in use but the shellac doesn't go off as readily as it does with ethanol. Longer shelf life.
As for spraying. Like brushing it takes a much longer time to harden, trapped solvent. It does harden eventually though. Less working time, much more waiting time.
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Alan Carruth
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Alan Carruth » Thu Aug 24, 2017 5:21 pm

I've been thinning my 2# cut shellac with acetone: one part shellac mix to two parts acetone. It's THIN. However, the acetone helps the alcohol dry out faster, and you end up being able to body up more quickly. It's not as dangerous as it smells; you do have enzymes for dealing with acetone, but you still want really good ventilation for this.

In my mind the root cause of mud on the surface is not too much pumice, but too much shellac. I start out with a bare wash on the wood, and very little in the pad when I'm filling. The shellac is only there to bind the pumice together, so the sign you need more is that there's loose pumice on the surface. It's easy enough to add shellac, but hard to remove excess mud.

Jim Frieson
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Jim Frieson » Sun Aug 27, 2017 4:41 am

the two basic ways of polishing are :

Oil on first . all over , as thin an oil as possible , never a boiled oil or a drying oil , then wiped off but not scrubbed off .
Vegetable oil OK , doesn't matter , but thin mineral oil is much better .
In this process there is no shellac used , no wash coat , no other gum or resin , they would be just an impediment .
Spirit , oil , and pumice , linen for rubber cover or raw coton rubber without cover are the materials . Cotton cloth is useless .
When the wood is filled , only then is shellac used , bit by bit , and pumice is abandoned and not used .
If a heavy coat of lac is desired , pumice may be used to smooth it out , but if so , it is done with a spirit rubber , without any shellac .
If this is mastered it produces a fine finish . It is very difficult to use on spruce or cedar.
It is very fast and a guitar can be polished up in less than a day .
It takes time to harden up . It is rare to find anyone who uses it but was common in the antiques trade .
I have used it on guitars .

No oil , only shellac . Padded on , sprayed on , brushed on , whatever . Then polished out in the end . There are a lot of varieties of process .
Oil not used untill there is enough lac to prevent oil getting to the wood .
In any case oil should me spread as soon as it is applied to prevent it going through in a spot . Especially if the finish is thin .

But if oil does get in the wood and make a splotch in an otherwise shellacked finish there is only one remedy . Remove the finish .
A pain but that s how it is .
I always found acetone pretty good at washing oil out .

Pinewhisp
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Pinewhisp » Tue Aug 29, 2017 9:32 pm

Thank you again to everyone for the advice. At this stage I have been successful at cleaning the walnut surfaces with naptha and sanding down to the bare walnut with dry 220 and 400 grit (400 grit alone was not cutting it), and re-cleaning with naptha. I've followed with preliminary shellac sealer rubbings and the walnut surfaces now look clean and even again. My walnut surface irregularity seems to have been conquered! Unfortunately I've noticed a lesser echo of the oil-contamination problem on my spruce top as well, which will require more careful treatment to avoid sanding ebony binding and dark rosette material into the light-colored spruce. I may rely more on solvents (alchohol and naptha) to decontaminate the spruce top. Not sure yet. As I begin again with the routine of French Polishing I really want to thank everyone again for the advice--I'll be sure not to over-oil this time.

Kevin

Pinewhisp
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Re: French Polishing Newbie Mistake: Is there hope?

Post by Pinewhisp » Tue Aug 29, 2017 9:33 pm

Make that 220, 320, and 400.

simonm
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Re: French Polishing Newbie Mistake: Is there hope?

Post by simonm » Wed Aug 30, 2017 8:57 am

Pinewhisp wrote:
Tue Aug 29, 2017 9:32 pm
…. At this stage I have been successful at cleaning the walnut surfaces with naptha and sanding down to the bare walnut with dry 220 and 400 grit (400 grit alone was not cutting it), and re-cleaning with naptha. ...
An older German professional maker now dead who bult guitars for close on 70 years told me he never used finer than 220. (European numbering). 120 was his "go to" if I recall correctly. On the other hand his generation routinely used cabinet scrapers for every conceivable task. A brilliant tool once you get the hand of it. Definitely worth learning how to use.

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Re: French Polishing Newbie Mistake: Is there hope?

Post by Alan Carruth » Wed Aug 30, 2017 6:47 pm

A scraper, of course, leaves a different surface than sanding, particularly on a softwood. The scraper tends to compress the soft earlywood and cut away the harder latewood. Later, the earlywood swells back up, and you're left with a sort of corduroy surface. Violin makers do it this way because that's what Strad did: you'll never get into trouble in the violin world for doing what Strad did. Guitar buyers tend to prefer the more level look of the sanded surface. You can, of course, level with the scraper and then sand to make it smooth; that's what I do (except on violins).

I've used drying oil on the wood without problems. I avoid using oil on the wood on guitars because it adds mass and damping, and the whole point of FP, IMO, is to do that.

My understanding has always been that 220 grit sandpaper leaves scratches that are smaller than the structural features of the wood, so there's no point in going any finer.

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