finish thickness

[Gavin Baird]

To all,
I have been interested in the various opinions of builders in their description of their finishes. There has been a great deal of emphasis placed on the minimum thickness of that finish. What I would like to hear from the various builders is what do they shoot for in film thickness and how they determine it. If in fact film thickness is of importance then I would assume that there is some way of determining it. How do you all out there do it? Gavin

[sunburst]

First of all, I finished every instrument that was produced by a mnaufacturer for six long years. That about took care of any sanity that I might have had left, (and left my fingers feeling about 40 years older from sanding) but it also gave me a feel for how much finish I could get away with, without buff-through problems. I don't even think about how thick the film would measure, I know I can't practically go any thinner without problems, and I just do it by eye and by feel, from experience.

[Neil Dean]

Like John, I also have experience in a production shop environment. #Level sanding finishes was on of my regular duties while I was gainfully employed by Rigel. #You just start to develop an intuititive feel for the film thickness after a while. # #Occasionally, someone else would have to do the spraying and that was when we usually ran into problems. #

I currently use an oil varnish which is an entirely different animal than the stuff we were using. #Its far easier to level out which is nice, but you can't drop fill imperfections. #Basically, your surface preparation needs to be perfect. #What I like about the oil is that once you get it leveled out, you apply a thin film of frech polish to achieve a gloss. #You don't have to worry about burning through with the buffing wheel. #After the level sanding stage, you actually add a minute amount of finish, rather than remove it. # So I'm sorry Gavin but I haven't a clue as to the actual thickness of my final finish. #I guess I can only emphasize that the better your surface preparation is, the thinner that your finish needs to be. #

Neil

[Hans]

Ditto here. Put too little on, and besides the burn through problem you have folks calling you up saying the finish is wearing off too fast. It's really a feeling based on past experience.

[HoGo]

I haven't as much experience as the guys who posted before me, so I had to learn when to stop.
What I did is that whatever color, ground finish went on instrument went on a scrap piece of maple and spruce. I used oil varnish and I applied a layer to that scrap piece with each layer on instrument and level sanded together with instrument (actually before I touched the instrument, so any sand-throughs were on a scrap...). When I was feeling there's enough varnish I scratched the test piece with my fingernail to see how much varnish there is, and decided from there.
Quite simple, I'll do it again when I'll change my varnish recipe...
Adrian

[Mario Proulx]

I measure my guitar's finish at the bridge area when I remove the finish from there. I'll try to get a small, clean chip, and measure it with a micrometer. By this point, it's now more habit than anything, but I still do it. I've measured anywhere from a little under .002" to not quite .005", after levelling and polishing. Since I use the same finish on mandlins, and the same amount of coats, I'd expect the same final thickness range.

As everyone's said, we get a feel for it after a while, but I'll be the first to admit I still "cut through" way too often...

[sunburst]

I'll be the first to admit I still "cut through" way too often...
When I did it every day, I seldom sanded through. It's been years since I did it every day, and I find that it's like everything else; practice goes a long way toward making you better at things. Now, I have to use a little more thickness, because I'm ...well...out of practice, and I'm more likely to sand through.

The thing to aviod is using extra thickness as a substitute for good tecnique with the sandpaper.

[amowry]

John (or anyone else), what do you use for sanding pads? I've been using scraps of heavy leather, erasers, and corks, but I've been curious about what other people use.

[Gavin Baird]

Andrew,
I use 1/8" leather and/or 1/8" soft rubber sheet for compound curves. For flats, flat ridgid surface backed up with the rubber sheet.

Back to the thickness thing. There are many things that control thickness of a finish. Prep. of the surface, type of material, viscosity and method of application. In a previous post I indicated my findings regarding film thickness and the results. This was done to give me a little more confidence in the final outcome of the finish and to insure that the finish was a thin as possible without major sand throughs.
I was really asking if anyone had ventured into this area of investigation and if so what their findings were.
I know that it is a temptation to apply more rather than less to insure success however this usually leads to excessive thickness.
With lacquer I know that to spray, using my equipment, that by controlling the vicosity accurately that I can achieve .001" per coat on a repeatable basis. With Varnish, due to its much lower viscosity, the film thickness is much, much less and requires many more coats ....Gavin

[sunburst]

Andrew, I use quarter sheets of sandpaper, fold them in half, then fold that in half. I put a piece of plain paper in the center of the fold, where the two gritty sides are facing. If I want more stiffness, I use stiffer paper, like in a business card, or a cereal box.
I use rubber sanding blocks for flat surfaces, like peg heads, but I don't use wood blocks, unless I have to. They're too hard on the joints in the fingers, hands, and wrists.

The added benefit of the paper between the fold of the sandpaper is; the grit in there is still sharp when the outside surfaces are worn out, so I can reverse the fold, and keep sanding until the sandpaper is completely worn out. I hate to throw away sandpaper that I haven't used, so I don't want to wear it out by rubbing the surfaces together.

[amowry]

That's a good idea, John. Thanks!

[Luthier]

FWIW I spent a year as the "Finishing Department" for LoPrinzi Guitars back in the late 70's. #I did it all from the final sanding to a wash coat to the paste wood filler and then the sealer. Four coats a day for three days with sanding between every four coats and then it sat for a week before it was sanded with 320 and buffed out. #No wet sanding I might add. #I know there are other ways to do things but this method had served me well for a long time. #I can't tell you how many mils thick the finish is but it works. #.........and you do the same thing day in and day out....it gets old....but the nice thing is the nitro has left no bad effects on me, no bad effects on me, no bad effects on me, no bad effects on me, no bad effects on me, no bad effects on me,

Don

[PaulD]

Andrew, I use quarter sheets of sandpaper, fold them in half, then fold that in half. I put a piece of plain paper in the center of the fold, where the two gritty sides are facing. If I want more stiffness, I use stiffer paper, like in a business card, or a cereal box.

<SNIP>

The added benefit of the paper between the fold of the sandpaper is; the grit in there is still sharp when the outside surfaces are worn out, so I can reverse the fold, and keep sanding until the sandpaper is completely worn out. I hate to throw away sandpaper that I haven't used, so I don't want to wear it out by rubbing the surfaces together.
John; An alternative to sticking the extra paper in there is to add a tear to your quarter sheet and and interleave the surfaces so that you always have grit against a back. I don't know if this JPG will be viewable in MandoCafe resolution, but it may describe it better than I can in words. I illustrated this with a full sheet in mind for another presentation, but I do the same thing for 1/4 sheets when I need a smaller sanding pad. You still end up with access to the full sheet, first by flipping the pad and then by refolding so the inner sheets are on the outside, but you never have grit against grit and trash half your paper.

Paul Doubek

[sunburst]

Well, Paul, that's probably not too new of a trick for this old dog, and I can still put the paper in there if I want the pad stiffer.

[PaulD]

Certainly not a "new" trick, even if it's new to you... I found it in the '70s in a boat building/finishing book that was published in the '50s. I've been using it ever since and I'm always surprised that I don't see more people do it. I see a lot of folks just fold the paper over with nothing in between and ruin the inside grit! http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif

pd

[Darren Kern]

Sunburst warned me that as a beginner I may not be able to tell when I'm about to burn through, and he was absolutely right. #Using oil varnish, I burned through in several places, and I thought I had the finish on pretty thick. #Fortunately they were tiny spots that I could fix/hide easily. #But I think this reinforces the "by feel, by experience" school of thought.