What cured finish chalks up with lacquer thinner?

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Travis Brown
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Joined: Wed Oct 31, 2012 12:42 pm

What cured finish chalks up with lacquer thinner?

Post by Travis Brown »

Was prepping to dropfill a ding on a Kronbauer. Was sure the finish was nitro, but when I dropped a bit of lacquer thinner into the spot to melt the cracked finish in the ding before dropfilling, the finish chalks up when the thinner dries up. I tried on an inconspicuous spot to confirm and get the same chalking, and maybe sorta softens the finish, but not like nitro would. The chalking just polishes off.

Anyone know offhand what this result is trying to tell me? I can't get ahold of Trevor Kronbauer to confirm.
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Barry Daniels
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Re: What cured finish chalks up with lacquer thinner?

Post by Barry Daniels »

I once bought some cheap lacquer thinner from HomeDepot and it left a chalky residue behind. However, if your finish is nitro, the thinner should slightly melt the lacquer and leave a visible etching where the thinner sat. I wouldn't be surprised if your finish is something else.
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Clay Schaeffer
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Re: What cured finish chalks up with lacquer thinner?

Post by Clay Schaeffer »

I think Barry may have found the problem. If you are using the wrong lacquer thinner the solvents may be flashing off in the wrong order. This article explains it better than I can:
https://thefinishingstore.com/blogs/new ... er-thinner

Alan Carruth
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Re: What cured finish chalks up with lacquer thinner?

Post by Alan Carruth »

Note that mixtures of solvents can end up evaporating faster (or at a lower temperature) than 'pure' solvents. The mix of 95% ethyl alcohol and water evaporates at a lower temperature than pure alcohol, which is why Everclear is 95% alcohol rather than 100%; the mix boils off faster than pure ethyl. This is an 'azeotropic' mix. Adding something else to such a mixture might mess up the way it evaporates. This is an aspect of chemistry I don't know as much about as I'd like to . I use a 2:1 mix of alcohol and acetone in French polishing, and it evaporates very fast.

Travis Brown
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Re: What cured finish chalks up with lacquer thinner?

Post by Travis Brown »

Thanks for the responses.

Yeah, it's definitely not a nitro finish. At first glance I thought it was because of a couple small finish checks. (And what I thought was softening lacquer wasn't.) It's not TruOil - (which I wouldn't expect for a production guitar). Maybe a poly, or some sort of WB or conversion lacquer?

It's not cheap thinner. It's a "professional grade" (says right on the can!) pure product from a commercial paint shop. I've used it on other woodworking without issue, and used it on other instruments (that were nitro). Talked to Mohawk (I am using their NC lacquer) and they gave it thumbs up. I'll try some other thinner to test Barry's suggestion. But I tried some flash tests on other surfaces in same environment and comes away clean. This test on the USB key is a 10:1 thinner/nitro, which produces same chalky result on the guitar.

It's weird. Here's another brush-on over some light scratches in another area after flash off. Same mix as on the USB key. Just polishes away with a finger. ¯\_(ツ)_/¯
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Travis Brown
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Re: What cured finish chalks up with lacquer thinner?

Post by Travis Brown »

So...finding other anecdotes and rationale of this. Comports with Barry's experience, though seemingly not limited to cheap thinners. Sunnyside is a respectable product.
I found this out when thinning high build nitrocellouse lacquer to get it to flow better using Sunnyside brand and had the surface film flashing off too soon and ending up with white chalky bubble like dots, wrinkles, flashing or dry flashing and things like that.
I found the thinning was likely causing the surface film to film over too fast trapping solvents trying to escape.
https://www.painttalk.com/threads/lacqu ... st-1664583

Though this explanation is a bit odd when I was getting this using straight thinner. There wouldn't have been solvents to trap. Unless some reaction with the wood. Odd it happens on some surfaces and not others. Got the same result on a known NC finish.

Will pick up some other thinners today to try. I have used this brand of thinner in the past, but perhaps formulation has changed. I don't see it really mattering since the chalkiness dissolves away with the next application of dropfill, and final finish will polish away. Just unsightly. Guitar top looks like it had a acne breakout.

Question for those in the know - I sometimes see reference to lacquer reducers, which never made sense to me. In other finishes, a reducer is a mix of solvent and e.g. urethane. I don't see that being the case with lacquer reducers. Mohawk told me that for their lacquer, even though they label their solvent "lacquer reducer," thinners and reducers are interchangeable.

Clay Schaeffer
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Re: What cured finish chalks up with lacquer thinner?

Post by Clay Schaeffer »

Question for those in the know - I sometimes see reference to lacquer reducers, which never made sense to me. In other finishes, a reducer is a mix of solvent and e.g. urethane. I don't see that being the case with lacquer reducers. Mohawk told me that for their lacquer, even though they label their solvent "lacquer reducer," thinners and reducers are interchangeable.

If you look at the "reducer" MSDS sheet it looks pretty similar to other thinners. Calling it a "reducer" might be a marketing thing to make you think you are getting something more than a "thinner"

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