Why We Like Shellac
A finisher who's unfamiliar with shellac gets advice on its use, advantages, and limitations. November 5, 2013
Question
I design and make furniture in Panama, and we are using a wood called Laurel negro (Nectandra megapotamica). We are making table tops and when applying the second coat of varnish sealer, it begins to wrinkle and crawl in certain spots. Is there are solution to this without having to sand back down to bare wood?
Forum Responses
(Furniture Making Forum)
From contributor M:
Once this happens, you're out of luck. What's your finish schedule? Perhaps there is an inherent flaw in your process that can be addressed.
From the original questioner:
Thanks for your response Contributor M. I have re-sanded down to bare wood, wiped with thinner, gave two coats of lacquer and then three coats of varnish. New question: why does the wood's oil rise to the surface?
From Contributor P:
Why are you applying varnish over lacquer? Could your problem be lack of compatibility between the two finishes, rather than the wood? Have you tried using shellac between the two finishes to guarantee proper adhesion, or tried shellac first on the bare wood?
From the original questioner:
I started applying a lacquer undercoat after the problem arose, just using varnish. There is no shellac in Panama. I thought lacquer was somewhat of a shellac substitute.
From Contributor P:
Shellac and lacquer are two completely different products and act in different ways. I would look on the internet for shellac suppliers who would be willing to ship to Panama. Woodworkers Supply in the US has a large selection of shellacs. A good ready mixed product is Zinssers Seal Cote, which is a pre-mixed dewaxed shellac which might be worth a try. Also, try asking some of the finishers on the Professional Finishing forum for suggestions on a different finishing schedule.
From Contributor C:
As stated, Woodworkers Supply carries shellac. I buy blonde, dewaxed shellac flakes from them and mix my own. I use Denatured Alcohol (DNA). DNA is available at the big-box stores in the paint department. You may be able to find this locally. Shipping DNA, or ready-mixed, liquid shellac, would probably be expensive since they are flammable. Also, shellac flakes are lighter than a gallon of shellac which will also keep shipping costs down.
One gallon of DNA to one pound of shellac flakes will give you a "one-pound cut." This has worked well for me as a sealer coat. By the way, I used dewaxed shellac since I also use water-borne finishes over the shellac. If the shellac still has wax in it, adhesion problems will occur. If you do this professionally (which I am guessing you are), ask Woodworker's Supply to set up a commercial account for you. The pricing is different for commercial accounts.
From Contributor I:
I agree with the others about shellac, it is an indispensable product and can be used for lots of things. Have a few pounds shipped to you and you won’t ever look back. Almost any alcohol will dissolve it so you can use grain alcohol as the solvent. Grind up the flakes fine, mix with alcohol and let it dissolve. Dilute it till it’s like apple juice and use as a wash coat, apply with a spray gun, brush or rags. Be careful of drips and fuzz from the rags. Dries quickly and is great for all kinds of compatibility issues.
From Contributor P:
A couple of basic points which might help explain shellac's use to you: shellac is a great sealer when there is a possibility of a finishing problem. In addition, and most importantly, shellac sticks to virtually anything, and anything sticks to shellac. You can use it to seal in oily woods, or as a barrier coat between incompatible finishes, such as varnish and lacquer. When applying a new coat of finish over an unknown earlier finish, a coat of shellac will guarantee adhesion.
Same thing if there is a possibility of silicone contamination on the piece - it will stick. Unfortunately, it lacks the alcohol and heat resistance of varnish or lacquer, which is why it isn't recommended as a table-top finish, but on more decorative pieces which are not subject to a lot of wear, it can be a beautiful finish. It dries fast and hard, and will polish out to a beautiful luster.
