Official Luthiers Forum!

I used zinsser spray can shellac as seal coat on the back, sides and neck of the guitar before starting to french polish. Clearly this was a bad idea as the spray shellac was either old or bad as it has remained tacky. I have french polished over it and even with the french polished coats it will leave an imprint if I leave the guitar laying on a T-shirt. I don't have this issue with the top of the guitar as I didn't use the zinsser shellac on it. I've been using the de-waxed shellac I mixed up from flakes for french polishing.

Any ideas before I scrape all the finish off the back, sides and neck?

Better to cut your losses and get back to bare wood. Skip any Zinsser shellac. Lots of folks say Seal Coat is identical to shellac flakes but my experience tells me it is way inferior. Seems like you should be able to remove the sticky stuff with straight alcohol and start again with flake shellac.

_________________
Stay with the happy people.
--Reynolds Large

This is a good example of the desirability of testing before applying. An extra step, I suspect, but it can save a lot of cleanup.

_________________
Peter Havriluk

use flake shellac the stuff in the can isn't as good
I grind my flakes in a coffee grinder 1 oz to 8 oz is 1lb cut
good seal mix don't use the spray it doesn't work well for guitars

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Agree with the above replies, but for completeness it is really easy to spray the seal coat on too thick. A wash or seal coat before a French Polish feels dry within moments and they are not really thick enough to imprint.

_________________
http://www.Harvestmoonguitars.com

I've been using the Zinsser rattle can shellac to keep handling scuffs and fingerprints off my soundboards before assembly without any problems - - - in a light coat.

_________________
Peter Havriluk

Did you use the de-waxed variety of Zinnser? I've used Zinnser for years both under nitro and for French polish (cut to 1 lb) with never a problem. Just used it to seal two 1950's tweed amps I built.

Zinsser Bulls Eye spray shellac is dewaxed blonde shellac - apparently, wax interferes with the mechanics of the spray process, so at least their spray product has been dewaxed since 2015 when I discussed this with them. I also asked about shelf life and was told that similar to Seal Coat, their extended life dewaxed amber shellac in a can, three years was maximum shelf life assuming the usual cool (not too hot/too cold) storage environment.

Zinsser shellac products are date-coded for product batch as follows:

6 or 7 digit alphanumeric code beginning with a letter, with second character the last digit of the year of manufacture, the third character either a number for month of manufacture (if Jan-Sep) or a letter corresponding to the first letter of the month if October, November, or December, and the fourth and fifth characters the day of the month.

For a date code of R01101, the batch date of manufacture would be decoded as January 10th, 2020. For R1D023, the batch date of manufacture would be decoded as December 2nd, 2021. I don't recall what the leading alpha character referred to, or the additional digit or digits after the date code, but I found Zinsser's product information line to be helpful.

A good practice with any finish or adhesive consumable coming into the shop is to determine date of manufacture and write this on the package or transfer container in plain language if - like Zinsser or Franklin - a code is used versus actual date. For products with a reduced shelf life after opening, be sure to also note the date when the parent container was first opened and a 'check or discard by' date for materials like Enduro-Var or Titebond.

You can conduct a film scratch test on shellac by applying a bit of product to a clean, dry glass surface, allowing the film to dry for a minimum of 12 hours, then using a coin or similar tool to attempt to scratch the dried film. If the film dents or curls off the surface in a flexible film instead of being scratched by the coin (you should see dried film debris along the scratch path, with the tested area powdering or fracturing into fine chips), the shellac mix is too old to use for finishing.

_________________
For the times they are a changin'

- Bob Dylan

Mixing up shellac doesn't have to be a time consuming pain. The problem comes in when you dump some dry shellac into alcohol. No matter how finely you grind it, it settles to the bottom and you have to keep mixing it up over a period of a day or two to get it all to dissolve. The better way is the 'tea bag' method that I got from an artist's handbook. Measure out the amount of shellac you'll want, don't grind it, and tie it up in a cloth bag. Old T-shirt material works well. Using the wire tie suspend the bag near the top of a wide mouth jar and pour the alcohol over it. Have the bag positioned so that the alcohol comes part way up it. Cap it, and set it aside.

As the shellac dissolves the solution drops down to the bottom of the jar, being a little denser, and forces the less dense alcohol to the top. You can see the stream of dissolved shellac dropping down. You can make a useful solution of shellac this way in only a few hours, and there's no need to mix it since gravity does the work for you.

At one point I had some relatively light colored seedlac. That stuff can have a lot of dirt and bug parts in it, so it usually needs to be filtered. I found that the T-shirt teabag was fine enough to capture all of that, and also the wax, which seems to be coarser particles in this un-processed stuff. So long as I didn't squeeze the bag the wax all stayed in it. Sadly, the wax doesn't separate as well with the lighter colored and more processed shellacs. It seems to me that the more processing the shellac goes through to lighten it the less tough it is.