Advice on Finishing Top

The Achilles' Heel of Luthiery
Zen
Posts: 301
Joined: Sat May 26, 2012 12:35 pm
Location: Ireland

Advice on Finishing Top

Post by Zen »

Hi All
I have started finishing my dreadnought top so I applied 2 coats of shellac sanding sealer and then lightly sanded before applying one coat of shellac using the recommended french polishing method. The top looked fine after the sanding sealer but the french polish shellac coat has showed up little flaws that I never noticed before.
I have tried to photograph what I consider to be little scrapes below the bridge area and to the left of the bridge but they may not show up to everyone depending on the computer you use and also my photography skills are nil. I will post a few pics anyway and hopefully someone will be able to see what I am seeing here in the workshop.
Any info on where to go from here would be appreciated or will these little scrapes disappear after several coats ?

Thanks
Rusty
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RUSTY
Zen
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Re: Advice on Finishing Top

Post by Zen »

Last few pics
Thanks
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RUSTY
David L
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Re: Advice on Finishing Top

Post by David L »

I'm no finishing expert (especially French Polish) but normally when I'm applying finish to an object and scratches appear, the only solution I have found is to go back to the sand paper, find the grit that eliminates the scratches and work back through grits to the final grit. A lot of time scratches (and other defects) are not visible until finish is applied, more finish will just make the scratches more shiny. French polish may be a whole different animal.
Perhaps Brian H. and/or some other folks with FP experience will enlighten you further on this problem while using the FP technique.

David L
Zen
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Re: Advice on Finishing Top

Post by Zen »

Thanks for reply, When I look at it again and from different angles it actually look quite good and unless I go searching for faults I generally will not see them but I'm wondering if its actually possible to have a spruce top that is completely free of little tiny defects. Maybe I am being too picky but hopefully the experts will chip in here as well. Thanks again for reply
Rusty
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Ben-Had
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Re: Advice on Finishing Top

Post by Ben-Had »

I can't see any. You aren't confusing scratches with medullary rays are you?
Tim Benware
Zen
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Location: Ireland

Re: Advice on Finishing Top

Post by Zen »

Ben-Had wrote:I can't see any. You aren't confusing scratches with medullary rays are you?

No . Unfortunately not medullary rays. Just annoying little lines or scratches that I really thought I had sanded away-sorry pics probably dont show them too well.
I know people dont pore fill the top usually but if I do decide to sand down again what would be a good product to get a level playing field again.
RUSTY
Zen
Posts: 301
Joined: Sat May 26, 2012 12:35 pm
Location: Ireland

Re: Advice on Finishing Top

Post by Zen »

OK, I made the tough decision to scrape everything back to ground zero and start again.

I am going to post a new thread as I dont want to confuse things here and thanks to all who posted
Rusty
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johnnparchem
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Re: Advice on Finishing Top

Post by johnnparchem »

Rusty. I read your FP post, I was hesitant to respond as you asked what the pros do, I am only an amateur. I have FPed a handful of instruments.

Easy question: just use the same shellac that you are going to FP with as a seal coat. I like to pad on a few wet coats of a 1 lb cut before I start my FP polishing.

I am not sure what your understanding of FP is but when done correctly it is many many layer of very very thin shellac. I charge a pad with shellac, sometime alcohol and sometimes oil and work the instrument until the pad goes dry. For each charge I am laying down a batch of coats for any given spot. I do as many of these "sessions" until I am happy.

Reading you posts and looking at the pictures that you posted I am not sure what you are seeing. After reading your FP post I am not sure your expectations on how the finish should look at any point in the process is correct.

Some times with spruce if you are sanding in the direction of the grain you can create ridges between the light and dark grained woods as they sand at different rates. Before I switched to a random orbital sander for the top when I was at 180 grit I would sand horizontal to the grain (only on spruce). That way I would not be developing ridges.

To answer another question:

When I look at it again and from different angles it actually look quite good and unless I go searching for faults I generally will not see them but I'm wondering if its actually possible to have a spruce top that is completely free of little tiny defects. Maybe I am being too picky

I think maybe your expectations are off for what the top should look like early in the finishing process. You might want to switch to the 5 foot rule, basically take a few steps back, and then see what the guitar looks like.
Zen
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Re: Advice on Finishing Top

Post by Zen »

Many Thanks for reply John and some great advise.
I always seem to panic at this stage of the build and the very mention of french polishing seems to give me the jitters ha ha.

One last thing--after a couple of wet coats of a 1 lb cut should I then french polish with that same cut or do I need to make a stronger cut as I do more sessions.
I know I really need to learn more about the process so thanks again
Rusty
RUSTY
johnnparchem
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Re: Advice on Finishing Top

Post by johnnparchem »

I usually do every thing with a 1 lb cut.

Here is how I do it. When finished I usually never sand the finish. I have polished it though.

I do 6 to 8 "sessions worth of polishing. A session for me includes charging the inside wad in my muneca. I actually dunk a wad of lambs wool in the shellac and squeeze most of it out. I place the wad in a 4 x 4 piece of cotton. Fold and twist until I can get a little shellac to squeeze out. I back off and blot the muneca off as I do not want anything resembling a drop or drip. For each session I lightly apply the shellac with long ways strokes. I keep applying until things get a little sticky. At that point I put a drop of oil on the muneca and use small swirls to cover the surface I am working on, usually a few times. As the pad dries out and it is harder to swirl, I try to get rid of the swirls with relatively hard long ways strokes until the surface looks pretty good and there are few swirls and very few streaks.

Usually after every one or two sessions I "spirit" off the finish by loading my wool wad with only alcohol I squeeze it out as I do not want drips. I put the wad in the cotton and apply long very light stokes across the whole surface. I want to wet the shellac but not have it dripping just enough so it is quickly evaporating, I keep doing this applying a harder force as the pad is drying out. This step helps level the shellac and remove the oil.
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