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That is caused by the level and buff process and the fact the surface A; is not flat & B; vibrates under the buffer. The center of the top is usually the thinnest finish left after everything is done. The films tend to get thin again at the edges leaving a section in between where it gets thicker. The more reactive and resonant your tops the greater the difference in thicknesses will be in these areas strictly because of how they vibrate and behave under the buffer.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

What I think some of you may be hearing in the FP finish is an increase in stiffness relative to mass from the pressure during application and the fact it packs the film tighter than it is when conventionally sprayed. Sort of prestressing the film perhaps? Because the only real difference is application... all the other coatings mentioned are sprayed. This would also by nature make FP the one finish with the most variables that could affect MOE and thereby perhaps tone. Especially when one considers the nature of the product is insect excrement.....

And since I have yet to hear anyone praise the miraculous sonic abilities of sprayed or brushed shellac I surmise the difference is more in the application if there is any at all.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Hi Terence,
Yes, I'm doing the oil varnish finish myself. I had terrible luck outsourcing my finish work in 2019 and it nearly destroyed my business. I haven't tried Brian's services yet but was considering it; however, the outsourcing process was just destroying me and I decided I just had to have control over this essential aspect of my guitars. This left me with limited finish choices - no polyester, no urethane, no nitro - just waterborne finishes or oil varnish. If the oil varnish didn't work out for me - the next option would have been EnduroVar. If that didn't work, then I would have just quite building until I could spray my own lacquer finishes.

I'm gonna do a writeup on my finishing process. I chose oil varnish because of tone, looks, and durability. The finish I'm using is about as durable a nitrocellulose lacquer (Murdoch's Uralkd 500). Oil varnish is challenging but the process is actual quite enjoyable - the Murdoch is pleasant smelling and nice to work with. The waterborne finishes can give you a nice gloss but I just don't like the way they look in comparison to every other alternative. I like it better than anything else I've had on my guitars up till now - it's a good solution for those of us that don't have spray booths.

Has anyone considered doing the prep work then having someone else spray the guitars (local body shop), then doing the sanding and buffing in house?

Ok that makes sense too because as we all know shellac is hard stuff and (well I assume anyway) that once the alcohol evaporates out of it then it returns back to it's glassy hard state.

So that brings us back to the thickness of the film.

Great thread!

Thanks, M

Looking forward to reading that, Simon. I've finished two guitars in U-500 and I really like it, but found it as you said...challenging. I love the way it looks, feels, and sounds, but I could definitely use some pointers to improve my application technique.

Save yourself the frustration caused by the body shop experience.

A cabinet shop would maybe do a decent satin finish if you aren't too fussy..... But this is a very specialized endeavor.

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Sorry MOE Margin of error? Dave

Modulus of elasticity, I think.

Thanks Don, should have worked that out. Dave

B. Howard wrote:
"What I think some of you may be hearing in the FP finish is an increase in stiffness relative to mass from the pressure during application and the fact it packs the film tighter than it is when conventionally sprayed."

I don't believe that. There's still solvent in the shellac after you've finished polishing, and it shrinks down as it hardens. Although it's thick and viscous, it's still a 'liquid' until all of the solvent is gone, and you can't pack a liquid. Has anybody actually measured the density of the film from FP, and compared it with flake shellac?

Martin Schleske's article in the 'Journal' of the Catgut Acoustical Society: Vol.3 #6, Nov. '98 "On The Acoustical Properties of Violin Varnish", pp 27-43 gives useful data on a lot of different finishes, including drying oils, shellac, and nitrocellulose lacquer. Briefly,
1) any finish adds mass,
2) a finish that sits on the surface, such as a resin, adds less than one that penetrates, such as a drying oil,
3)resins, such as nitro and shellac, can have lower damping than wood, particularly cross grain soft woods
5)drying oils have higher damping, and the damping of something like an oil-resin varnish will track the proportion of oil.

The penetration of a drying oil probably depends on the presence of low molecular weight components: 'stand' or 'sun thickened' oils, or things like 'Tru-Oil' should penetrate less, but that's just an opinion.

Cross grain spruce strips finished with shellac or nitro had lower overall damping and higher stiffness then unfinished strips. Other finishes were more 'neutral' or added damping, iirc.

I don't have any tone monkey proof of different finishes on acoustics,
but a while ago I was thinking about a wipe on poly finish for an acoustic,
because I had just finished a electric slab git with the General Finishes brand, and liked it.
I did a test on an 1/8" thick cutoff of African mahogany,
and decided against it, because the finish "piped" right through the grain holes in the mahogany.
I know wiping poly has more oil in it,
but it led me to believe it would affect tone.
Just saying.
Alan

Alan, I would be curious to know the results of your wiped on poly test if you did a spot cost of shellac first.

Hi. I did a test directly on raw wood, so I would definitely do a sealer of shellac first.
It was a long time ago, and I can't remember if I used the sealer of the same brand first.
I think it was called "seal a cell". Anyway, good luck on your quest for tone here.
Alan