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kerf prints after lacquer & humidity change
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=18415		 Page 1 of 1
Author:  micheal [ Sat Aug 16, 2008 10:47 pm ]
Post subject:  kerf prints after lacquer & humidity change
I have two dreadnoughts that I'm currently trying to finish. I moved from a low humidity climate to a high humidity climate. The mahogany sides were thicknessed to approx. .085" before bending. I imagine the sides could be as thin as .075" in some areas due to my finish sanding. They're finished w/ nitrocellulose lacquer (which is also very thin, already sanded through on the soundboard and near these kerf prints). The back is sanded and glued w/ a 50' radius form, and the top is sanded w/ a 15' form and glued w/ a 30'. I couldn't imagine that under the circumstances this tension could twist the kerfing like this.

Image

Image

Image

These are pictures taken during the level sanding of my sides. I would like to know if you guys feel I should reinforce this area w/ some more vertical strips?
Also should I just make my sides thicker in the future so I wont run into this problem?
Author:  micheal [ Wed Aug 20, 2008 11:12 pm ]
Post subject:  Re: kerf prints after lacquer & humidity change
can i get any advice??
Author:  Wes McMillian [ Thu Aug 21, 2008 2:49 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
I saw your original post but I guess I was unclear on the question and hoped someone else would chime in. Guess it just slipped off the radar.

Anyhow.... I'm still a little unsure about the "kerf prints". I do see in the pictures some low spots in your sanding and levelling process. And now, reading back, I see a reference to "tension twisting the kerfing" and have to assume the guitar was built in a low humidity area (as you stated) without any climate controlled conditions and moved to high humidity area - still with no climate control? If these assumptions are correct, the first stage would be to get some control on your RH and bring it into a more acceptable range.

Some more info would be helpful in getting you the answers you need. What RH was the guitar built in? Do you have a way to lower your RH where you are now? I assume this guitar is a personal build for you? If so (and, like others, I hate giving advice on limited information which could turn out to be misleading if the assumptions are wrong), I would try to dehumidify and give it a week or so. At that point, I would just accept that where the guitar sides are is where they'll stay and try to blend them in the best I could with a soft-backed sanding pad and not worry about level sanding. Finish, string her up and play!

Main lesson to learn is to find a way to control RH for future build. Again, assuming I haven't missed the whole question?
Author:  jackatwork [ Thu Aug 21, 2008 6:41 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
I don't think those are kerf/lining prints; from what I can see, the sides aren't level and the shiney spots are below the surface for the rest of the sides.
Author:  micheal [ Tue Aug 26, 2008 8:59 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
No, I have no control over the RH where I'm at now (yet), but the RH here is around 40-60% constantly. I guess getting a dehumidifier would help the situation. The guitars were built in a low RH (25-35%). I like the suggestion about dehumidifying for a week and work off of what I got. West, do you think I should maybe add some reinforcements (extra vertical strips) inside the guitars in these spots??

Yeah, a more stable RH in the shop would benefit me a whole lot.

Thanks,
Mike
Author:  Wes McMillian [ Tue Aug 26, 2008 9:20 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
My personal advice would be to not worry about adding side reinforcements, since the box is already closed. The primary reason for them is the prevention of cracks, which can result from impact (the dreaded "key crack") or drying out (which is more likely to show itself on the spruce top, I think). Since it was built in low RH, drying out should never be an issue.

But, yes, you'll definitely want some better RH control. If you live in high RH environment, it will be money well spent. I wouldn't try to dehumidify back down to the 25-35% range it was buit in, just down to the typical desired range in the mid 40's. Once it stabilizes there, I'd just blend everything in and finish it where it is. Then get started on that next one!
Author:  Michael Dale Payne [ Tue Aug 26, 2008 9:44 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
I to agree that the issue is caused by low spot in the rim. I am not so sure that RH in the shop lead to them but rather moisture applied to the wood during bending and was not fully cooked out, or moisture in the glue used to attach the linings may have cause them. I say this because all examples you show are at the edges of the rim. If RH in the shop was the culprit I would expect the low spots to be in the center of the rim.

The way I see it you have to sand the finish level trying to avoid as many sand troughs as possible. If you have some sand troughs spray 2-4 more coats allow to cure and level again.

I do not know if you checked to insure the rim was flat before you bound the box or not, but I always do. If I have low spots I will scrape them out prior to binding because if you use a pilot bearing on your binding bit it can lead to deep spots in the channel and then just this kind of finish leveling issue.
Author:  micheal [ Mon Sep 01, 2008 6:42 pm ]
Post subject:  Re: kerf prints after lacquer & humidity change
Thanks for the advice. The sides have been flat all throughout the build, but I do remember (when sanding the sides flat (before binding)) I sanded down to similar low spots like these. I did spray the sides kinda hefty with water before bending, and I did use titebond for the kerfing. Would you suggest to use another type of glue that isn't water based? Plus the humidity levels in the shop weren't stable.. :?
Author:  Howard Klepper [ Mon Sep 01, 2008 11:50 pm ]
Post subject:  Re: kerf prints after lacquer & humidity change
Check this thread on the UMGF. Style 45, from the Martin factory:

http://www3.telus.net/toddb/side1.jpg

http://www3.telus.net/toddb/side2.jpg
Author:  dunwell [ Tue Sep 02, 2008 7:30 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
Hmm, when you cut the binding slot, did you by any chance use a router bit with a bottom bearing? I had one that had a bit of a burr on the bearing and it left a track/line impression in the side that looks very much like what I'm seeing on your instrument. I just sanded back a bit to get rid of it but it sounds as if you are already about at your desired thickness.

Alan D.
Author:  micheal [ Mon Sep 08, 2008 7:44 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
Yeah, I used a bottom bearing rabbit bit. Not sure if this is the case. The plan is to get a stable shop, acclimate the guitars to the shop, & respray sides and sand throughs. I figure 40-45% RH is suitable for my situation.
Author:  dberkowitz [ Thu Sep 11, 2008 6:51 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
If you are actually seeing kerfing prints in your sides, you probably sanded them too thin and the change in humidity and/or flexing is telegraphing through.
Author:  JJ Donohue [ Thu Sep 11, 2008 7:23 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
Howard Klepper wrote:
Check this thread on the UMGF. Style 45, from the Martin factory:

http://www3.telus.net/toddb/side1.jpg

http://www3.telus.net/toddb/side2.jpg


Great post Howard!

Soooo...Martin's high end guitars can be released from the factory with high and low spots. These are almost impossible to detect without a straightedge I imagine.

So it begs the question... should high end custom guitars be rejected for similar "defects"?
Author:  Dave White [ Thu Sep 11, 2008 7:36 am ]
Post subject:  Re: kerf prints after lacquer & humidity change
JJ Donohue wrote:
Howard Klepper wrote:
Check this thread on the UMGF. Style 45, from the Martin factory:

http://www3.telus.net/toddb/side1.jpg

http://www3.telus.net/toddb/side2.jpg


Great post Howard!

Soooo...Martin's high end guitars can be released from the factory with high and low spots. These are almost impossible to detect without a straightedge I imagine.

So it begs the question... should high end custom guitars be rejected for similar "defects"?


JJ,

Or alternatively since many players use Martin as the "benchmark" by which all others are judged, maybe high end custom guitars should be rejected as "defective" unless they match those Martin profiles :D
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