Some Notes on Finishes,

provided by Jerry Johnson

(The information was obtained from the book: “Understanding

Wood Finishing”, author Bob Flexner, Rodale Press)

 

Finishes:

 

There are 3 types and they are classified by how they cure: evaporative, reactive and coalescing.

Evaporative finishes have solvents of alcohol, acetone and lacquer thinner.  The main kinds are shellac, lacquer and wax.

 

Reactive finishes have thinners that are mineral spirits and naphtha and are often listed as petroleum distillates. The main kinds are linseed oil, tung oil, oil-varnish blends, wiping varnish, varnish, and polyurethane.

 

Coalescing finishes use glycol ether as the solvent and water as the thinner.  The main kind is water-based finishes.

 

Classification:

 

TYPE

FINISH

TYPE OF CURE

Film

Shellac

Evaporative

Film

Lacquer

Evaporative

Film

Varnish & Polyurethane

Reactive

Film

Water Base

Coalescing

Film

Conversion(conversion varnish and catalyzed lacquer)

Reactive

Penetrating

Oil and Oil-Varnish Blends

Reactvie

 

 

What happens during curing?

 

Evaporative finishes are made up of long, spaghetti-like molecules that pack together and interlock when the solvent evaporates.  When the solvent is reintroduced, the molecules separate, and the finish returns to liquid form.  Evaporative finishes thus have solids that have been dissolved in a solvent.  They cure from the bottom up, so the top is the last part to cure.

 

Reactive finishes undergo a chemical change during the curing process.  The molecules link together in a tinker-toy-like network that can’t be broken by reapplying the thinner.  Chemists call this reaction crosslinking or polymerization.  There are 2 categories of reactive finishes: those that cure by reacting with oxygen and those that cure when a chemical catalyst is introduced (much like the way epoxy cures).  Catalyst curing finishes are called conversion finishes.  They are not as well known or commonly available as other finishes and are mostly used by professional finishers.

Coalescing finishes, typically water-based, are more complex than the other 2 types.  They are sort of a combination of the other two types.  They are tiny dispersions of a cured finish emulsified in water.  Glycol ether is added so the droplets can cure as a film.  As the water evaporates, the tiny droplets come together and the solvent, which evaporates more slowly than the water, softens the drops so that the molecules of a drop can interact with the molecules of another drop.  It will take several weeks to achieve a maximum bond.

 

Solvents and Thinners:

 

A solvent will dissolve a cured finish, a thinner won’t, it thins a liquid solution.

 

SUBSTANCE

SOLVENT FOR

THINNER FOR

Mineral spirits, naptha, and turpentine

Wax

Wax, oil , varnish

Alcohol

Shellac, lacquer, water base

Shellac

Lacquer thinner

Shellac, lacquer, water base

Lacquer, catalyzed lacquer

Glycol ether

Shellac, lacquer, water base

Water base

Water

---

Water base

 

Lacquer:

 

Pros:  very fast curing; excellent clarity and depth; excellent rubbing properties

 

Cons: high solvent content that is toxic and flammable; only moderate resistance to water, water vapor, heat, wear and solvents; poor film build up – only about 10-20 % of the film you apply will remain as a solid film

 

Why people like lacquer:

·       Easy to apply

·       Very quick drying time; 3-4 coats in one day

·       Ease of repair

·       Exceptional film clarity, producing the appearance of great depth

 

 

Oil Finishes: See table