Shellac and Alcohol

Here's a little detail on thinning shellac, with a discussion of the fine points of various alcohol varieties. February 12, 2007

I'm a finish carpenter first, part time finisher second. Recently, I've been discovering the usefulness of shellac. I always thought of it as an old school finish for antiques and the like.

1. Can it be used as a sealer for exterior applications (front door)? I read somewhere that shellac is a naturally UV resistant finish.

2. I've been using Seal Coat - is there a retarder that will slow the dry time? I'm having trouble maintaining a wet edge when brushing.

3. Is Behlen's/Mohawk (same co.) "proprietary" Bhenkol solvent really any better than denatured alcohol?

Forum Responses
(Finishing Forum)
From contributor T:
1. Not a good idea. Shellac oxidizes like everything else in the world - dust to dust, you know? Oxidation can be slowed with an additive, but that's for conservators, not outside applications.

2. Seal Coat is a 2 pound cut which is about max for brushing. If you cut it to 1 1/4 or 1 1/2 lb with a good thinner, you should have more time. A very good brush helps too.

3. Absolutely. Alcohol absorbs water from the atmosphere. Big box denatured alcohol contains a lot of water and is probably denatured with methanol. A good grade of denatured alcohol (Behnkol) still contains some water. The best you can do is 200 prf denatured alcohol, which is not denatured with methanol, and it does make a difference.

From contributor M:

Ask your supplier if he can get you an anhydrous alcohol. If you really want an alcohol that contains absolutely no water, this is it.

From contributor T:
You could buy anhydrous alcohol, I suppose, and you'd get no water, but what are you getting in the way of alcohol? Methanol? Ethanol? Isopropanol? You would probably have to go to a chemical supplier and pay for reagent grade stuff. Why not go to Touchup Depot and get their 200 prf ethanol (aka shellac thinner) or some of their good denatured alcohol?

From contributor G:
By the way, water in the alcohol used to reduce shellac will also act as a retarder, being as water evaporates less quickly than alcohol. I've mixed shellac with 70% drugstore isopropanol.

From the original questioner:
That's exactly what I was thinking - the more solvent in a "purer" formula, the faster the shellac is going to set up? 70% isopropanol? Are you using iso specifically as a retarder?

From contributor T:
Shellac will dissolve in any alcohol, including watery alcohol, but if you don't know and can't reproduce the solvent mix, you'll get pretty unpredictable results. Water will slow the evaporation rate. In fact, you can dissolve shellac flakes in alkaline water (hat makers varnish) and it will take forever to dry. However, I've found that with a 1 1/4 # cut and a good brush, evaporation rate is not an issue and with a good solvent, it works the same way every time. The Shellac Net web site includes a good article on brushes by Don Williams.

From the original questioner:
Thanks for your replies. I checked out the site - great info. They do sell a specific shellac retarder to aid in brushing. I'll first just try cutting to 11/4 cut and see how that works.

I'm curious to know though what is in their retarder. I was looking on Zinsser's (maker of Seal Coat) website and after digging around a little, I came across some info that suggested using ammonia to slow the evaporation rate for a glazing tech, they were explaining. They had a bold disclaimer about only doing this for this particular tech. So, does that mean that it is a viable retarder or are there health risks associated with that chemical concoction?

From contributor G:
That is interesting. My guess about their retarder is some type of glycol ether like Mohawk No Blush, Cyclosol 53, etc. To answer your question, I didn't start out to use 70% rubbing alcohol as retarder; I was working in a very hot place with practically zero supplies available. The retarding effect was serendipitous. For what it's worth, you can dissolve some types of pre-mixed water-based dyes in alcohol and they will then go into the shellac for use as a toner.

From the original questioner:
Interestingly enough, Zinsser's website has info that says "ammonia retards the drying of shellac." Anyone have any thoughts on this? Is it harmful to spray/breath anymore than other coatings?

From contributor R:
This is also quoted on Zinzzer's site (at the same place):
"NOTE: Do NOT use ammonia to prolong the dry time of shellac for any other application! It is intended only for use with tinted or pigmented shellac that is being applied as a wiping or pickling stain."

From the original questioner:
You're right - I assumed that was some sort of disclaimer they had to put there. That does not change the fact that they suggest using it as a retarder for shellac. I'm not advocating using ammonia to retard shellac - I've never tried it.

From contributor T:
I admire your tenacity. If you're interested in learning, here's a simple science quiz for you: Add a known and equal amount of shellac to 2 identical cups. To cup #2, add an amount of alcohol that is equal to the amount of shellac and stir. Let them air dry. Which cup will dry out first? (Cup #1.) Why? (Because #2 contains more solvent and more solvent takes more time to evaporate.)

Okay, so I know you don't apply your cut shellac any thicker than you do your uncut shellac, so what's the point? Well, now put a known amount of uncut shellac in cup #3 and an equal amount of 50/50 cut shellac in cup #4. Which will dry out first? (Cup #3.) Why? (Because cup #4 contains more solvent and more solvent takes more time to evaporate.)

So now how would you retard the drying time of your shellac? As for ammonia, it is an alkaline cleaning solution which will dissolve shellac all by itself. In fact, I believe Zinnser recommends it for cleaning brushes. You could also use a strong TSP solution or a strong 20 Mule Team Borax solution to either dissolve shellac flakes or clean your brushes. If you really need a lot of open time, this will give it to you. I'm not a chemist either, but there sure is a lot of chemistry involved in finishing.

From contributor R:
I would suggest you also check out the Homestead Finishing site. This is a quote from there:

"Butanol - because of the 4 carbon atoms - you have more isomers. So you can have butanol or isobutanol. Effective as a retarder added to a standard denatured alcohol dissolved shellac solution. It's a very disagreeable smell, but used in small amounts, it's not too bad. An isomeric form of butanol is added to Behlen's Bekhol shellac solvent."

"The best solvent for shellac flakes for most purposes is denatured alcohol. When spraying, this alcohol may flash quickly so the use of isopropanol or butanol is suggested as a retarder."

It's a great site if you're interested in learning many things.

From the original questioner:
Thanks for indulging me on this subject. I think I have enough info to play around with and see what works for my particular situation.