Official Luthiers Forum!

I've heard an awful lot about witness lines and waterborne lacquer, but have never seen a clear example of what they may look like. I've been working on the assumption that these are them, but thought I'd ask to make sure. I only really ever see these lines over darker colors, which I guess makes sense, as there is no grain pattern to obscure them.

I think so.

Yup when you sand through one layer that's the tell tail sign. One of the reasons I gave up on water base

Looks like witness lines to me. Which water borne lacquer is it? What stage are you at in the sanding/polishing in the photo?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Yup
I’d also like to know which finish you’re using. I’ve used em6000 on 2 instruments so far and haven’t had this issue


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That's them. Train your eye and you'll see them on the spruce and lighter colors too!

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Brian

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

https://www.howardguitarsdelaware.com/

I've been brushing on Brite-tone lacquer. This is after wet sanding (400, 600, 800, 1000, 1500, 2000, 2500) before buffing and polishing.

This is a headstock repair I did on a Seagull 12 string. The lines are really only a little visible over the black, the rest of the neck seems to be fine. Now that its super level I may do another couple coats of clear on top and not sand so vigorously. I've started wet sanding with mineral spirits instead of water, I seemed to get a lot more witness lines when wet sanding with water.

I'm super new to finishing, and have found it to be probably the steepest learning curve of all the luthiery related tasks I've learned over the past few years. I've been hesitant to learn with things other than water-based as I work out of my house and have kids around, and who knows how much longer things like nitro will even be available. The big plus for me with this stuff is I can brush on a few coats in the morning in my studio/workshop and teach guitar lessons in the afternoon and not worry about paint fumes bothering any of my students.

Another question occurs to me. Are witness lines the result of my poor technique, or just the nature of the beast with waterborne lacquer? From reviews I've read of this product people experience a lot less (or no) witness lines than they do other water-based products. And it is supposed to behave similarly to nitro.

They can happen whenever you sand through coats of a finish that doesn't burn into previous coats. Without a chemical bond across coats, the witness lines are the "between" layer if you will. Usually doesn't happen with nitro lacquer or traditional shellac. I've had them appear with varnish, though a light pass on the buffer took care of those.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Witness lines are not the nature of the beast for water borne lacquers. Maybe for some of them, but in my experience, witness lines haven't been a problem using EM6000 water borne lacquer. I've sanded through to wood and no witness lines. I've repaired a sand through and sanded through again and no witness lines. I've repaired EM6000 a couple years after it was originally applied by applying new EM6000 and had no witness lines appear from level sanding and polishing.

I haven't used Bright-tone lacquer, but maybe it's sensitive to the length of time between coats. If you go over a certain length of time between coats, a new coat may not "burn in" to the previous coat. Did you have any long periods between coats?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I also use em6000 and the only time I have seen what you show in your photo is when I sanded through to shellac underneath...

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Steve Sollod (pronounced sorta like "Solid")
www.swiftcreekguitars.com

Yes? I think so. I left it as much as a week between coats I think. I don't have my records in front of me right now.

That might be where they come from. I would see if you can find an info sheet from the manufacturer and see if there are recommendations on the maximum time between coats.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

If this Seagull headplate is not going to be glossy, then sanding/buffing the final application may very well be unnecessesary. Blow on the last application and let it dry. If it looks good, stop. My guess, susceptible to correction by folks with experience.

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Peter Havriluk

Not necessarily true. 2K urethane and UV polyester, neither "burn" into eachother and neither will show witness marks between layers when done properly. This is a water based finish phenomena more than anything, although sloppy work will cause witness lines even in shellac or nitro.

The reason traces back to the same thing that causes the lack of clarity typically seen in these finishes, especially over darker substrates. And it was already pointed out this is where the witness lines show most as well. The mechanics of suspending acrylic, polyesters and other plastics in an aqueous solution causes an initial polymerization not present in other technologies and this is the reason for this.

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Brian

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

https://www.howardguitarsdelaware.com/

Yes, witness lines as others have said.
I use water-based Enduro Var, and have repaired a finish sand through (headstock/neck joint area) after more than a month's drying with no witness lines after buffing.
I dry sand EV BTW.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

You are sanding too much, which is common with inexperience. You don't need to use all of those grits. And you are starting with too low of a grit. Everyone has their own schedule, but you could probably simplify the grits to 1000 and 1500 and then polish.

Okay cool, less sanding is definitely worth a try. I'll put another couple coats of clear on the black peg face making sure to not leave it too long between coats and then sand with just those two grits.

I guess it goes without saying that to get away with less sanding you've got to start with an impeccably prepared substrate?

When I'm working on especially the middle/latter stages of an instrument I keep hearing this voice echoing from someone on this forum -
"Preparation is key to finishing"

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Yes! No kidding. But having come to luthiery with virtually no previous woodworking experience I had no idea the weight of that comment. More than anything else it seems between preparation and measuring there are constantly layers being pulled back and new horizons of doing it better and more thoroughly.

I hear that!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

A couple more coats of clear, with a little over 2 hours between application. Let it dry. Gently and thoroughly dry sanded with 1000 and 1500, then polished it out. Seemed to work much better!

Nice!

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Just the ticket!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I'll just add that spray technique is critical, too. I feel like I'm finally laying down smooth coats that require little sanding.

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Tony Thatcher
Bozeman, Montana