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The sealer coat of shellac
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Author:  SnowManSnow [ Fri Apr 27, 2018 2:22 pm ]
Post subject:  The sealer coat of shellac
I’m getting ready to do a FP finish, and am in the process of doing the seal coat. My question is... how much perfect should this coat be? I mean, after applying 3 coats levels JT then repeating, shoulders I have a perfectly level coat with no glossy spots”?
I’m having trouble accomplishing this... maybe I’m not putting enough on to take off to GET level.
My method has been with a pad, center being cheese cloth:)


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Author:  Goodin [ Fri Apr 27, 2018 6:05 pm ]
Post subject:  Re: The sealer coat of shellac
I'm not sure what you mean by " levels JT then repeating, shoulders I have a perfectly level coat with no glossy spots?".

There is no need for the seal coat to be perfect since the shellac will melt into the subsequent layers. The first several coats are just to get some shellac on there to work with. The shellac will get spread out and levelled out during bodying sessions. I just wipe on the seal coat and base coats with a paper towel, without much thought of it being perfect. I just try to make sure I dont have too much shellac on the towel that I might leave a heavy sag area that would have to be rubbed or sanded out. I wipe on as many coats as it takes for a sheen to start to build, typically 3-5 coats, then I start in on the bodying sessions with a muyeca (however that is spelled!, sorry my Spanish is rusty!). I used to use cheese cloth and linen for this but now I use a large cotton ball and an old (but clean) white t-shirt.

Here is a recent restoration I did of a rare 1920's Epiphone Recording Artist 5-string banjo with a french polished shellac finish:

Attachment:
IMG_2188 (Small).JPG

Attachment:
IMG_2319 (Small).JPG
Author:  DennisK [ Fri Apr 27, 2018 7:08 pm ]
Post subject:  Re: The sealer coat of shellac
There's too much variety in what is called French polish to give a straight answer :) But in general I'd say don't worry too much about it. Keeping dust/fuzz out of the shellac is more important. A dust grain will accumulate shellac like a pearl, resulting in a little mound that's hard to remove without then becoming a crater that won't go away either.

Another thing is that if you really slop shellac onto bare wood so it has time to soak in before it dries, it will give a darker, more wetted appearance. If you want a flatter appearance with color more like bare wood, then use a fairly dry cloth for the initial coat.

For the wet style, I keep the squeeze bottle of shellac in one hand, and cloth in the other, and just pour a pool of shellac and push it around. Usually needs to be scraped level afterward.
Author:  SnowManSnow [ Fri Apr 27, 2018 7:08 pm ]
Post subject:  Re: The sealer coat of shellac
I apologize for the confusion on my post! Yea that didn’t make any sense. My thumb / iPhone interface seems to be off tonight.
!


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Author:  SnowManSnow [ Fri Apr 27, 2018 7:40 pm ]
Post subject:  Re: The sealer coat of shellac
Dennis: one of the things I struggle with is finishing. I feel like I’ve made a nice body, but my finish skills haven’t developed yet... still very much learning. This won’t be my first FP attempt, but I’ve yet to finish one to my liking.
I have taken Tom Bills’ class and found it a great source of information, so I have worked in that method so far.
Repetition


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Author:  meddlingfool [ Sat Apr 28, 2018 2:40 am ]
Post subject:  Re: The sealer coat of shellac
I think it's fair to say that you should buckle your seatbelt re finishing.
Author:  B. Howard [ Sat Apr 28, 2018 6:34 am ]
Post subject:  Re: The sealer coat of shellac
Seal coat? After pore fill I just apply FP.... Watch my video here and I may answer a lot of questions for you.
https://howardguitars.blogspot.com/2015 ... l-way.html
Author:  jfmckenna [ Sat Apr 28, 2018 7:46 am ]
Post subject:  Re: The sealer coat of shellac
To speed things up I brush on a couple coats of shellac then level sand before doing the actual FP. The level sanding should take out the majority of the glossy spots but you don't have to be obsessive about it.
Author:  B. Howard [ Sun Apr 29, 2018 6:15 am ]
Post subject:  Re: The sealer coat of shellac
jfmckenna wrote:
To speed things up I brush on a couple coats of shellac then level sand before doing the actual FP. The level sanding should take out the majority of the glossy spots but you don't have to be obsessive about it.



Brush on your first coats and you may as well just brush it all on.......

You are trying to put a hard polish over a softer base at that point. The magic of FP is in the pressure during application......
Author:  jfmckenna [ Sun Apr 29, 2018 10:03 am ]
Post subject:  Re: The sealer coat of shellac
I've tried finding some actual science behind the pressure and the hardness of the shellac coat but never could find anything. I'd love to actually see something to it but my simple minor in chemistry has me questioning how pressure can alter the chemical structure of shellac to make it harder. My understanding of it is that there is a solvent in the shellac that evaporates and the finish shrinks. But if anyone has some papers on that I'd love to read them. Perhaps the pressure is really heat? I don't know. I know the masters have said this for hundreds of years now and I should not be one to argue with them but they've also been known to say other things that science says otherwise. Suffice to say in the end everyone has their own unique way of doing just about everything finishing notwithstanding. Brushing on the base coat works for me.
Author:  doncaparker [ Sun Apr 29, 2018 11:37 am ]
Post subject:  Re: The sealer coat of shellac
I only brush coats on before pore filling, to give that process enough shellac to work with.
Author:  Alan Carruth [ Mon Apr 30, 2018 1:08 pm ]
Post subject:  Re: The sealer coat of shellac
I agree with jfmckenna: I don't see how FP can end up denser than brushed shellac. As he points out, FP shrinks back over time as the last of the alcohol evaporates, so it's hard to see how it could be 'compressed'. It would not be too hard to check out: make up samples of wood that have either padded or brushed shellac on them, and check the weight gain against the film thickness. If you really want to be exact you could apply the finish to glass, to avoid the possibility of the wood absorbing different amounts.
Author:  Michael.N. [ Mon Apr 30, 2018 5:05 pm ]
Post subject:  Re: The sealer coat of shellac
I think it's a myth. Same with the pad creating friction and heat, hardening the shellac. If that's the case why not just warm the shellac long before applying it (as in 1704 varnish) - that will generate far more heat than friction.
I've done a lot of French polishing and brushing of shellac over the years. The advantage of using a pad is that it helps to fill pores/pin holes far better than brushing. It's also easier to maintain a smoother brush free surface, easier to keep the finish as a thin film. Brushing is quick though - less working time but more overall waiting/hardening time, quite a lot more. I suspect that many people think padded shellac is harder because they simply don't give brushed shellac enough time to fully harden.
Author:  B. Howard [ Tue May 01, 2018 6:16 am ]
Post subject:  Re: The sealer coat of shellac
Alan Carruth wrote:
I agree with jfmckenna: I don't see how FP can end up denser than brushed shellac. As he points out, FP shrinks back over time as the last of the alcohol evaporates, so it's hard to see how it could be 'compressed'. It would not be too hard to check out: make up samples of wood that have either padded or brushed shellac on them, and check the weight gain against the film thickness. If you really want to be exact you could apply the finish to glass, to avoid the possibility of the wood absorbing different amounts.


One word..."PRESSURE". French polish is supposed to be done with pressure! The pressure almagates the shallac and packs it together tighter than it will on it's own.... you are supposed to be increasing the density of the material during application. Same way they pack loose asphalt into a hard road you can drive on.......

I say it every time, that FP has gotten a reputation for being a finish that doesn't hold up well only because most people are doing it completely wrong or very poorly.
Author:  B. Howard [ Tue May 01, 2018 6:26 am ]
Post subject:  Re: The sealer coat of shellac
jfmckenna wrote:
I've tried finding some actual science behind the pressure and the hardness of the shellac coat but never could find anything.


From the very first page of my google search..... "The effect of hydrostatic pressure predominates the molecular relaxation dynamics through an increase in the polymerization rate. " https://aip.scitation.org/doi/abs/10.1063/1.472948
Author:  Barry Daniels [ Tue May 01, 2018 9:55 am ]
Post subject:  Re: The sealer coat of shellac
Brian, I don't think hydrostatic pressure is relevant to the pressure you apply to a pad.
Author:  Michael.N. [ Tue May 01, 2018 11:14 am ]
Post subject:  Re: The sealer coat of shellac
Shellac is a pretty hard material as it comes. Try scratching a flake or a button, that will give you an idea. It stands up to my playing extremely well but I have had an instrument returned after just 3 months and the finish had worn through. In fact there were signs in a few areas that showed it almost down to bare wood. It's obvious that some players can wear it through in next to no time. I assume it's due to alkalinity. My own personal guitar was a brushed on finish, done 8 years ago. The finish intact save for a few dings and a some minor bit of dulling. Then again I've known for a long time that a shellac finish and myself are good friends.
Author:  Alan Carruth [ Tue May 01, 2018 1:03 pm ]
Post subject:  Re: The sealer coat of shellac
"Molecular relaxation of the various states formed during the course of growth of a linear‐chain polymer by addition reactions of the amine group of cyclohexylamine with the epoxy groups of a diepoxide has been studied..."

Are those the resins in shellac? Is pressure on the pad the same as 'hydrostatic pressure'? They spoke of pressures 'up to 200 bars' and more. One bar is ~14#/sq. inch. How hard are you leaning on the guitar? Does pressure increase the amount of polymerization, or just the rate, which is what they talk about? And so on.

FP is favored for two reasons, so far as I can tell. One is that it's the only finish that can be applied in a sand storm: with skill you can get it to look good without the need for much dust control, and without a lot of expensive equipment. The second is that it goes on thinner than any other finish, and so loads the soundboard less.

As natural resins go, shellac is pretty hard, and quite a lot tougher than most. But .002" (.05mm) of any resin is simply not a very hard surface. No matter how hard and tough it is it's not going to resist denting very well. Shellac is also well known to be soluble in alkaline water solutions, which, for some folks, seems to be 'sweat'. That's why some people can go through it so fast.

As I said above, since there is alcohol in the film when you stop polishing, I don't see how pressure can pack the molecules closer together on the surface unless it does induce them to cross-link faster. If that's the case it ought to be possible to demonstrate it. Until I see some data using shellac I'll remain skeptical.
Author:  SnowManSnow [ Mon May 07, 2018 12:16 pm ]
Post subject:  Re: The sealer coat of shellac
As far as pressure goes... I was under the assumption that more pressure simply aided in achieving a thinner application


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