Official Luthiers Forum!

I am trying to level sand a guitar that I sprayed with Zinsser sealcoat last weekend, and it is still gumming up badly. The shellac is only on as a sealcoat between z-poxy and KTM-SV, but it didn't spray on dead flat (or, more accurately, I was unable to spray it dead flat). I can scuff it with a scotch pad but that doesn't level it at all, so I will be shooting topcoats on an unflat surface, less than ideal.
Is there a solution to this? I had thought that using shellac as a sealer was real handy cause it dried so fast...
TIA

In the past, I've had problems with 'old' or 'bad' shellac gumming up instead of sanding to a powder. I have an idea (which I haven't tested) that very humid conditions make the problem worse. So those are possibilities....
One idea which I picked up years ago is to put some of your shellac on a piece of glass (old, clean window glass) and let it dry. Try sanding that and see if it still gums up. This can eliminate some possibilities in your troubleshooting.

Sometimes hanging the guitar out on a nice breezy day will get the finish hardened up quicker than in the shop. I have my neighbor (who is into working on old cars) 'trained' to check my clothesline for guitars before shooting primer in his driveway!

The good news is that you can remove the shellac fairly easily (if necessary) with paint remover/stripper or alcohol-soaked paper towels, though it is messy. Let the guitar dry out well and start again. I've done this several times when having problems with French Polish/shellac!

I'd definitely get that gummy shellac outta there (or hardened) before proceeding with your KTM.
Cheers
John

I mix my own.
That way i know how fresh it is,and that there's nothing
else in it that may impede drying.

www.schellac.net

_________________
Mike Collins

Yeah mix your own. Whatever doesn't get used, throw it away after a month or so.

If it doesn't dissolve quick, as in a couple hours when finely ground, or most of it in half a day if in flake form, again throw the bag away as it must be old. Seems that the flakes expire too after a long while.

Better be safe and lose a couple $ than having gumming problems or a some soft overall finish that will come back to haunt you.

_________________
Build log

+1 on the mix your own.

To build on Alexandru's post - you can strain out whatever doesn't dissolve and can be pretty assured that what did dissolve is good. I usually let it dissolve over night.

Mixing your own shellac is really easy. Don't buy more shellac than you'll used is a couple years a time. You'll be good.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

I recently French polished a cedar top using 3 year old Zinsser Shellac sealer mixed at 1# cut with Everclear. I have never had gummy shellac in the 8 years and a number of FP guitars done during that time. Perhaps it's the Everclear or perhaps it's the fact that I transfer Everclear into 4-6 smaller bottles when I buy a large bottle. This assures that water absorption is kept to a minimum. Excess water in the mix could definitely result in gumming. Testing, as someone mentioned is a good practice but an even more important practice IMO is to keep your shop humidity controlled.

I am not adverse to tossing out old shellac but let's not forget about maintaining basic shop conditions as well. In my case, I believe it turned out to be more of a factor in achieving consistent results.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

One of the things the the main luthier in the shop insists on is the best de-waxed shellac flakes we can find. When we mix it up, we have to date it, and he is fussy about how 'leftovers' are stored. He's convinced that off the shelf shellac is inferior, if only because you don't know how long it has been on the shelf. Stored in a cool dry place we get about 6 mos out of it.

The root cause of all this was gumming up in the past, plus almost severing an artery getting a jar open that had glued shut.

As an aside, we also use everclear as opposed to DA. Mostly for the smell but also because the shellac flakes seem to dissolve a bit more uniformly.


I am very interested in hearing how the KTM-SV works for you, or perhaps I should just put my query in a new thread.

Thanks for the replies folks. I suppose RH could be an issue. Though my workshop is controlled my hanging area ain't. How long should it take for shellac to be sandable?

Lucien...We have some quite advanced luthiers on board who have touted the benefits of Zinsser dewaxed Sanding Sealer (or a European equivalent) in the can. They are as fussy and particular about the materials they use and have reported similar results as those where the finest and freshest flakes were used. I would say we have a difference of opinion and I differ because I have listened to their reports and then compared and made decisions based upon my own trials. I would suggest that others do likewise...there's enough disagreement out there to justify the effort.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

A few times I got a textured bumpy surface while french polishing, God knows why. I kind of suspect that I did too much straight with the grain motion. The important thing is I could always sand the bumps the following day, with 1000 grit, with absolutely no issue.

_________________
Build log

It sounds to me that you didn't allow sufficient time for the shellac to dry between coats, and/or you sprayed it too thick for each coat, and/or you have too much thickness. How many coats did you apply, how much, and in what timeline?
The Zinsser seal-coat works fine, even if the can was open a year previous.

_________________
Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Laurent,
those could all be issues as well. I used preval sprayers which can gop it on fairly thick. I waited 1 1/2 - 2 hrs between coats...

Also to build on your build...

I grind the flakes in a coffee grinder, and I use a small lab magnetic stirrer I got on ebay (around $25)... I now get NO un-disolved flakes. I then strain it twice and it leaves almost no residue. It might be overkill, but it works beautifully.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Yes, how many coats per day, and how many coats total?

_________________
Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Only four coats as it's just the sealer between epoxy and KTM-SV, all coats one day...

I mix mine by settling the flakes in a jar, adding the alcohol so that it completely covers the flakes, and letting it sit for two hours before I touch it. Just letting it sit is important because the alcohol will first make the flakes clump - THEN it breaks down the clumps. If you try to shake it or stir it prematurely, clumps may stick to the side of the jar above the fluid line......makes a mess of it.

Then I turn it a few hours in a cheap rock tumbler. Harbor Freight $30.

Just had to find the right pickle jar to use for the rock tumbler but this works very well. No residue whatsoever.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

I don't have any experience with pre-mixed shellac. I've been happily mixing my own for the past twelve years. In his book on wood finishing, Tage Frid describes the risks of using old shellac. He recalls using up an old batch on some pantry shelves for his own house. Years later, the shellac was still sticky. With that advice in mind, I've always been careful to use freshly mixed shellac--a couple of months after mixing is my limit. I do a lot of French polish work, so that's pretty easy to manage. In this past year, I got a crash course in what happens when shellac flakes are stored too long, or stored in high heat and humidity. I received a batch of flakes from my regular supplier that looked a little different. The flakes themselves looked slightly cloudy. I mostly use platina shellac (paler than super blond). The cloudiness troubled me, but I needed shellac, so I mixed up a batch. It wouldn't dissolve. No amount of stirring would get rid of the gelatinous floaters swirling around in the mix. Gentle heating didn't help. I filtered out the floaters, and French polished a couple of guitars. The finish crazed. My supplier was happy to exchange the bad batch, but the replacement was no better. At this point, I really needed shellac, so I tried mixing up some super blond flakes that had been sitting on my shelf for several years, ever since I switched to platina. These too, wouldn't fully dissolve. Was it the alcohol? No, I was using the same can of SDA 23A alcohol that had been working fine previously, and is now working fine again. I talked to a friend who does French polish work for a local guitar manufacturer. They buy their shellac flakes from the same source, and they had the same problem. I'm using a different supplier now, and I haven't had any more problems. I do examine the flakes more closely now, and pay attention to how easily they dissolve. I've heard that the paler shellacs are more prone to this problem. I have some very dark flake shellac that has been in my shop for 10 years. It still mixes easily, and dries rapidly.

Eric:
There's somethin' bout your avatar that jes ain't right!

_________________
The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

If you use shellac as a seal coat, you put way too much on. A couple of light coats and a light scuffing is all it takes.
BTW if spraying shellac I do like a lacquer schedule, no more than 3 coats/day, then leveling the next day. It sounds like your coats are uneven, too thick and haven't had the time to cure properly. FYI Zinsser seal coat has some sort of preservative in it, so it doesn't go bad like "mixed" shellac does.
The Preval is a useless piece of junk IMO. A cheap HVLP gun and compressor will go a long way toward achieving consistent results.

_________________
Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

That's my hero, Hiram Bream, Country Western music's foremost lutenist.

X3 on everything Laurent says!

I will add a few things...

LET it dry a LONG time... A medium-thick coat of shellac can take 3-5 MONTHS to dry to the point of not being mushy under the surface! Trust me on this..... French polish finishes on Classicals are famous for needing up to a YEAR to fully "Cure" -- when they are significantly less apt to fingerprint and scratch on contact.....

I would seriously reconsider using Shellac in-between 2 Epoxy coats -- you are asking for trouble... The residual Alcohol and absorbed water may not be able to dry -- leaving you with a mushy middle coat between 2 harder coats... Dent and scratch and craze city....

Last thing -- what are you level sanding with? I never had any luck Dry sanding shellac... Paint thinner (The High-Odor Smelly stuff) is the lubricant of choice when wet sanding Shellac that is not Ancient (meaning Less than 50 years old....) DO NOT USE WATER!

All that said -- Zissinger Shellac seems to be pretty good stuff overall.. My #1 Retop Poject is finished in Zissinger Bullseye Spray shellac... and it is an extremely durable finish once it finally cured out.... which took close to a YEAR!

Thanks

John