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Blotchiness on end grain - what to do?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=39301		 Page 1 of 1
Author:  MaxBishop [ Sun Feb 10, 2013 4:07 pm ]
Post subject:  Blotchiness on end grain - what to do?
In finishing my last guitar I experienced some dark blotches on the heel. This was on a spanish cedar neck I finished with brush-on varnish. I suspect that it could have been avoided by properly sealing the neck prior to varnishing. Can anyone give me a tip as to how to avoid this in the future?

Thanks,
Max
Author:  ernie [ Sun Feb 10, 2013 4:36 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
several coats of shellac underneath the finish. act as a sealer
Author:  B. Howard [ Sun Feb 10, 2013 4:52 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
Spanish cedar is known for extractive bleeding of resins. I usually clean the stuff really well with DA prior to finishing to remove any residual resin.
Author:  DennisK [ Sun Feb 10, 2013 5:18 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
Shellac doesn't darken Spanish cedar endgrain too much, so that should work for a sealer.

Thin hide glue soaks in even less, so it's good for sealing endgrain on lighter woods like Port Orford cedar that darken a lot even with shellac.

Run tests with both in combination with your finish, and see which looks best.
Author:  Ken C [ Sun Feb 10, 2013 8:41 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
He Max, You may also consider sanding that area to a finer grit, and make sure the whole area is consistently sanded.

Ken
Author:  Stuart Gort [ Mon Feb 11, 2013 1:20 am ]
Post subject:  Re: Blotchiness on end grain - what to do?
Sand and wet and sand and wet...and so on until the grain stays down.

The thing that worked best for me when faced with uneven absorbtion into end grain on a very light Port Orford Cedar was a water based acrylic sanding sealer. It took several coats and resands due to grain raising but when it was done the color was very pale yellow....and even...no matter what the orientation of the grain.

I learned to repeatedly wet with water and sand smooth in that episode. If I had started out just wetting with water and sanding I would have probably only had to apply one coat of the sealer before topcoating...maybe.

But if you use shellac to seal...still do the wetting and sanding first. Take it to a fine grain with the papers and avoid using a thin cut since alcohol is a surfactant and will seep deeper into end grain than edge grain.
Author:  MaxBishop [ Mon Feb 11, 2013 6:55 am ]
Post subject:  Re: Blotchiness on end grain - what to do?
Stuart,

How thin is "thin"? Would you recommend using Zinsser right out of the can? I believe it comes as a 2 lb. cut.

Thanks,
Max
Author:  Stuart Gort [ Mon Feb 11, 2013 12:33 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
MaxBishop wrote:
Stuart,

How thin is "thin"? Would you recommend using Zinsser right out of the can? I believe it comes as a 2 lb. cut.

Thanks,
Max


First...make sure the Zinsser is "dewaxed" shellac. Second...be dubious and patient enough to test it thoroughly on scrap material. Third...yes...out of the can is as thin as you want to ever want to use Zinsser...if that. Fourth...I'd defer completey to someone else who has actually done it with Zinsser. :)

The concept of "surfactant" is the key. A surfactant breaks down the surface tension of a liquid. Denatured alcohol is the carrier for shellac but since it's a surfactant it will have a "seeking" effect on a fresh, conducive surface. Kinda like when you take a felt tip pen to a piece of paper...the ink spreads out due to the surfactant nature of whatever carrier is used in the ink. So...if using any particular shellac has discolored your part...using even more alcohol will drive it deeper into the open end grain and discolor it even more.

Getting some super blonde shellac flakes and making your own is probably best. My first test would be a four pound cut. If that discolors the test piece I would only expect a two pound cut to be worse.
Author:  MaxBishop [ Mon Feb 11, 2013 3:34 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
Thanks to all for your advice, especially to Stuart for the detail info' on the action of surfactants and his experience with shellac as a sealer.

Max
Author:  Stuart Gort [ Mon Feb 11, 2013 6:40 pm ]
Post subject:  Re: Blotchiness on end grain - what to do?
MaxBishop wrote:
Thanks to all for your advice, especially to Stuart for the detail info' on the action of surfactants and his experience with shellac as a sealer.

Max


...or lack thereof. :)

Please note I tested shellac as a sealer on Port Orford Cedar (test panels only) and ended up using the water based acrylic sanding sealer on the actual guitar instead. There was no shellac based experiment that pleased me. Mind you...I was sealing around the entire edge of the body of an electric guitar. Since that edge shows grain at every angle, any discoloration was unacceptable...and there was a lot of discoloration on my test pieces with shellac. I ended up on the phone with a luthier who had made this beautiful guitar out of almost pure white Holly. I saw it on the internet and called him. He was kind enough to steer me toward the water based acrylics.

I have a fresh Bamboo guitar to seal that is going to present the exact same problem of uneven discoloration along the edge. I'll be experimenting with a sprayable catalyzed polyester sealer on this one. Test pieces first though...as always.
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