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| Question on Shellac/Nitro Adhesion http://luthiersforum.com/forum/viewtopic.php?f=10101&t=33430 |
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| Author: | Stuart Gort [ Sun Sep 04, 2011 2:23 pm ] |
| Post subject: | Question on Shellac/Nitro Adhesion |
I haven't done the testing of adhesion that I'm capable of. I have the stuff but the time involved is extensive and I'm doubtful this time would be well spent. I've used Zinsser Bulls Eye shellac under nitro but I wonder if there are certain brands of shellac that provide a noticably better bond with nitrocellulose lacquer. I've also used dissolved flakes but I only have amber tinted flakes at the moment and I need clear. With the Zinsser I got a tiny lifting of the nitro when I was reaming the tuner holes. I'm wondering if this is to be expected or if that indicates a problem. Zinsser indicates on the can that this stuff isn't for urethane topcoats but there is no warning about any other topcoat. In the aviation indusrty, the durability of any finish was almost entirely predicated on the bond strength of the finish to the substrate. So we spent a LOT of time figuring that out and got finishes that you could whack with a steel rod and the finish would follow the dent down into the impact without any sign of delamination. Being used to that standard I'm somewhat disconcerted to see this tiny lifting around a few of these tuner holes. Seems to indicate an adhesion issue in the making. What's normal...in terms of the finish lifting around a hole made after finishing? Is anyone used to seeing holes drilled through finishes without a hint of lifting? If so...what brands of shellac/sealer/nitro are you using? |
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| Author: | Mike Dotson [ Sun Sep 04, 2011 2:43 pm ] |
| Post subject: | Re: Question on Shellac/Nitro Adhesion |
IMO Zinsser, or any prepared shellac shouldn't be used under finish, or even on an instrument. I've used spit coats of shellac under nitro for color, but I use vinyl sealer both directly on the wood and on top of shellac before the clear. |
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| Author: | woody b [ Sun Sep 04, 2011 3:20 pm ] |
| Post subject: | Re: Question on Shellac/Nitro Adhesion |
Zinnser "Shellac" isn't wax free. Zinsser seal coat is wax free. I don't know if wax causes problems with Nitro or not though. I've used Zinsser Seal Coat under Nitro with no flakes when drilling the tuner holes..........actually I drill my tuner holes before finishing, but I don't get flakes around the endpin hole. I spray 2 coats of Zinsser Seal coat, let it sit overnight, then lightly scuff before spraying with Nitro. I haven't used Nitro in a couple years though. Nitro will adhere just fine without any type of sealer, but I had some dyed purfling lines bleed once. (arg!!!!). If the finish lifts around a hole I suspect they'll be more problems in the future. When testing new finishes, or combinations I cut a cross hatch pattern into a finished test piece with a sharp knife, then apply and jerk off some sticky tape. I believe the finish should stay. I also cut a piece to see if it de laminates where I cut it. I also put the piece in my freezer for a while, then in direct sunlight. In my mind this is checking flexiability and also assures me the expansion/contraction rates of the finish and sealer are compatable. A whack with a steel rod sounds like a good idea too. |
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| Author: | Rod True [ Sun Sep 04, 2011 3:47 pm ] |
| Post subject: | Re: Question on Shellac/Nitro Adhesion |
I'm with Woody and I've used the Zinsser's seal coat under nitro on 5 guitars with no issue. As Woody said, make sure you've let the shellac cure at least over night and give it a scuff with 400 or synthetic wool before spraying your nitro. You can also just use thinned lacquer as the "sealer", many folks do that with fine success. Or you can use vinyl sealer made by the sale company as the lacquer... |
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| Author: | bluescreek [ Sun Sep 04, 2011 5:30 pm ] |
| Post subject: | Re: Question on Shellac/Nitro Adhesion |
I stopped using shellac under nitro. This can sometimes be detrimental . I use only the manufacturers recommended sealers to avoid adhesion issues. Nothing is more frustrating when your client calls you to tell you the finish is coming off. |
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| Author: | B. Howard [ Mon Sep 05, 2011 6:01 pm ] |
| Post subject: | Re: Question on Shellac/Nitro Adhesion |
Shellac is great because it sticks to almost anything and any type of finish will stick to it. The stuff even sticks to the Formica bench tops, it will not scrape off, must be cleaned off with alcohol. I have not had a finish de-lam that I could trace back to a shellac bond coat failure, at least one I could actually blame on the shellac. With all that said, I agree with Todd on the vinyl sealer under lacquer. Vinyl sealer is engineered to work with modern lacquers and level sands a lot easier than shellac. |
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