Polyurethane, Lacquer, Shellac, or Varnish? What do I use? What’s the difference?

Article:

Your woodworking project is done. And maybe you’ve stained it too. Now you need to protect it with a topcoat. What do you choose? Maybe this information guide can help you...

from;
James King
King’s Fine Woodworking


Highlights: What are you looking for?

Fastest Drying Time:
Lacquer - 30 min
(Shellac - 1 hour
Waterborne polyurethane - 2 hrs
Standard Polyurethane - 24 hours)

Best for Crystal Clear Finish:
Lacquer
Waterborne polyurethane - close second place.

Easiest to Apply:
Wipe on polyurethane
Spray on poly from a can
Spray lacquer from a can
-All three are easy options

Easiest to Repair:
Lacquer
-Spraying on another coat partially dissolved the coat underneath, leading to perfect repairs.

Easiest to Clean Up:
Waterborne polyurethane
-Soap and warm water.

Most Durable:
Polyurethane
or Lacquer (close 2nd place)

Best for Outdoors:
Varnish
-Spar Urethane


-Detailed information on each Topcoat-

SHELLAC:
Shellac is a natural product. It is made from the dried secretions of the female Lac beetle. Once they are dried, collected, and processed, you can dissolve them in a solvent such as alcohol. And then it can be used as a finish.

Shellac dries by the evaporation of it’s solvent.

Shellac first came into widespread use for a furniture topcoat sometime in the late 1500’s. Coincidentally this was a period in history when the profession of a ‘wood finisher’ called a “Varnisher” became a distinct class of work, separate from a woodworker.

Prior to the 1960s shellac was probably the most popular form of topcoat. Shellac is also popular because you can put a a color or a tint into it. And it holds that color very well.

Shellac is very beautiful and forms a reasonably durable finish that can be high gloss in nature.

It does have drawbacks however. If you place a hot mug or a hot pan onto a shellac finish, a white ring can develop under it. It is also soluble in alcohol. So if you spill an alcoholic beverage onto it, The finish will start to dissolve. For these reasons we don’t recommend shellac for table top surfaces.


LACQUER:
Lacquer has been in use for almost 7000 years. And Chinese Lacquer-Ware has been found from the Neolithic Age. (Last part of the Stone Age) It was originally made from the sap of the “Toxicodendron vernicifluum” tree. Today it’s known as the Chinese lacquer tree

Modern lacquers however, were invented in the 1920’s and today are made via entirely synthetic means. It is extremely durable and one of the hardest of the topcoat finishes. It is stronger and better wearing than shellac and varnish. (Debatable as to whether or not it’s harder and more durable than polyurethane). Lacquer is also capable of producing an extreme high gloss finish. Lacquer is impervious to alcohol and most other common household liquids that might be spilled on it.

There are two classes of lacquer. Those that dry with solvent evaporation, (ie. the ones you buy in a big box home improvement store, such as Deft, Behlen, Rust-Oleum). And those that dry via a chemical reaction. These are called catalyzed lacquers. They are generally found at a specialty paint store, and have a very short shelf life.

Lacquer can be bought in all finishes; from matte to high gloss. It has the fastest drying time of any top coat, and thus it’s possible to achieve 3 - 4 coats in one afternoon.

Lacquer is the most forgiving of all topcoats. It’s easy to sand away a run. Or patch an area that got scuffed or simply didn’t get coated well. It is easy because application of a new coat partially re-dissolves the prior coat, and they adhere well and blend together perfectly.

Lacquer is the clearest of all topcoats. When you really want the beauty of the natural color of the wood to show through, it is ideal. However, on some species, lacquer can take away some of the depth and the 3D quality of the wood that is achieved with the amber-toned color warmth of polyurethane.

The very best way to apply lacquer is to spray it. This can be with a spray system. Or with aerosol cans. Brushing lacquer is available also. Note: brushing lacquer sprays on perfectly well! But, spray lacquer dries too fast to be brushed on.

There are drawbacks to lacquer however. Lacquer can discolor over a long time. Lacquer has a high VOC (volatile organic compound) content, which makes it dangerous to use without a respirator that has an organic vapor cartridge. A dust mask will not protect you from the vapors of lacquer.


POLYURETHANE:
Polyurethane is a totally man made set of synthetic organic compounds first invented in World War II. It is a polymer (poly = many, mer = part) made from the reaction of diisocyanate with a diol.

It is arguably the hardest & most durable of all topcoats. And in recent decades polyurethane, or poly (as I’ll call it for short), has become the go to topcoat finish for woodworkers everywhere. Nearly every finish & topcoat manufacturer has come up with a poly product for sale.

It can be bought in all finishes, from matte to gloss. And, it can be sprayed on, brushed on, or even wiped on. The wipe on is usually a thinner mix to allow for better leveling. If the Poly, is brushed on, a natural bristle brush is best to avoid bubbles that could form from a foam brush.

Polyurethane can come in two major forms. It can be oil based or it can be waterborne. The waterborne version of polyurethane is newer and has some advantages.

Waterborne polyurethane is made by combining microscopic particles of polyurethane small enough to maintain a colloidal suspension with water. It would be wrong to call this polyurethane water-based. The polyurethane’s base is still an organic solvent. It is just that this is carried by the water to make it easier in a number of ways. Waterborne polyurethane dries very quickly because as the water evaporates the microscopic particles of polyurethane are able to cure very fast.

Oil based Polyurethane has a slow dry time, usually about a day. It cures by a chemical reaction. And it must be sanded between coats to obtain a ‘tooth’ for the next coat to grab on to. The oil based poly gives a beautiful warm glow to the wood.

Waterborne poly has a quick dry time of about 2 hours. It also provides a clearer finish, it has a lower VOC, and it is easier to clean up. Just use soap and water.


VARNISH:
Varnish is often used as a generic name for topcoats. But it is actually a very specific product. This terminology difficulty is actually exacerbated by manufacturers calling their products by the wrong name for the sake of marketing. Companies in the past have freely called, shellacs, lacquers, polyurethanes, & oils all by the name Varnish.

Varnish today is usually an alkyd resin mixed with a solvent that cures to dry via a cross-linking chemical reaction triggered by oxygen molecules. It is not a lacquer, and not a shellac.

Varnish is related to polyurethane, in that Poly is a type of Varnish. Poly is harder and more rigid however, and has less oil in it. Generic Varnish, has more oil in it, and is more flexible. Chemically speaking, they are distinct and separate things.

Varnish is excellent for outdoor use, and generally impervious to saltwater, heat, cold, and UV light. Also being a cross-linked resin compound that is high in oil, it is very flexible and it can expand and contract with the temperature making it ideally suited to all weather environments.

It is quite durable. But not as durable as polyurethane. It does impart a warm glow to the wood and could certainly be used on indoor furniture. But outside of bathroom furniture, where humidity might be high, it would be uncommon to see varnished pieces inside.

Good luck on your next finishing project!


By
James King
King’s Fine Woodworking
February, 2018


SOURCES:
Polymer Science Learning Center, University of Southern Mississippi.
Journal of Chemical Education 69
Ullmann’s Encyclopedia of Industrial Chemistry
Journal of Biological Macromolecules
Germ plasm Agricultural Research Service
Rust-Oleum News Blog