I.       Why Finish

 

Protection

 

Finishes, to a lesser or greater degree, protect from moisture, oxidation, Ultra Violet, and abrasion.

 

Myth: wood is alive because it moves.

 

Fact: wood is not alive but the cell structure will  take up and give up moisture causing movement that will eventually deteriorate the structure of the wood.

 

Appearance

 

Finishes make the grain appear richer and deeper, enhance color, and bring out (pop) special grain features such as curl and bird's eye.  Someone once said the most gratifying part of a woodworking project is the application of that first coat of finish when the wood shows itself for what it really is.

 

Note:  some exotics are very dense and contain natural oils such that they can be polished to an excellent appearance without the benefit of finish ( cocobolo, ebony, African black wood).

 

 

II.               Preparation

 

General Sanding Protocol

 

Start with a grit just coarse enough to remove machine and/or hand tool marks (80-100g).  Sand only until all tool marks are removed and a uniform scratch pattern is achieved.

 

Proceed to the next grit and sand only enough to reduce the scratch pattern to the finer grit.  Brush or vacuum the piece between grits.  Although abrasive manufacturers say you can skip one grit, I don’t recommend it.

 

Final sanding should be done by hand with the grain at the last grit used on the power sander.  This ensures any cross-grain scratches or swirl marks are removed.

 

The final grit depends on the grain orientation and choice of finish.  If end grain or burl, sand to at least 400g. – let appearance be your guide.  If a stain or oil finish is to be used, proceed to at least 220g as stains and oils tend to pop scratches.  If a film forming finish (e.g. lacquer, varnish) is to be used, 180g is probably sufficient.

 

Finally, before applying finish or stain, spritz the piece with alcohol. As the alcohol flashes off it will pop any coarse grit scratches or tool marks that were missed.  Better to find them with alcohol than with stain or finish.

 

Flattening Panel Glue-ups

 

Start with a belt sander and a grit appropriate to the unevenness of the panel, i.e. the amount of stock that needs to be removed (60-80g).  Pencil or chalk marks across uneven joints aid in determining when the panel is flat.  Sand full diagonal strokes in one direction, then in the opposing direction until the panel is flat.

 

When flat, sand with the grain at the same grit until all diagonal scratches are removed.

 

Proceed through the grits with whatever type sander you prefer.

 

Special Sanding Problem for Turners

 

Sanding on the lathe while it is running in many cases produces cross-grain scratches.  Sanding at the last grit should be done with the lathe stopped and in the direction of the grain as much as possible.

 

Tip:  Before applying finish or stain, sprits the piece with alcohol or mineral spirits. This will 'pop' any coarse grit scratches/tool marks that were missed in the process of sanding to the final grit.

 

Final Thoughts on Sanding

 

If water-borne stains or finishes are going to be used, the grain must be raised and sanded at least once prior to application.  Thoroughly wet the wood with water and let dry over night.  Then sand lightly with worn 220.  That will remove the raised fibers without cutting new ones.

 

Grit scales are different between US and Europe.  US is CAMI and Europe is FEPA (P scale).  They're about equal up to 400g.  After that they drift apart (CAMI 800 approx. equal to 1500P).

 

Aluminum Oxide vs. Silicon Carbide - AO is used for sanding bare wood.  Silicon Carbide (black wet/dry) is used for sanding finishes.

 

Myth: sandpaper changes to a finer grit as it wears out. 

 

Truth: sandpaper wears out.  Think of one grain of abrasive with its sharp edges.  It cuts a well-defined scratch.  As the edges dull with use, is begins to tear as well as cut and requires more effort on your part.  Finally when all edges are dull, you're just burnishing the wood.  Using worn out sandpaper is either causing more work or affecting the result or both. One Exception:  sanding raised grain. 

 

III.    Choosing a Finish

 

Considerations

 

When choosing a finish, consider 3 things; durability, appearance, and method of application.

 

Durability – How and where the piece is going to be used dictates how much protection is required.

 

Appearance – Do you want a natural wood look or a surface film adding depth and sheen?  Do you want the finish to add color or not?

 

Application Technique – do you have the right environment and/or equipment for applying the selected finish?

 

Evaluating a Finish

 

Evaluation can be difficult because of inconsistent use of terminology.

 

If it says Tung oil it may or may not be an oil finish and may or may not be Tung oil.

 

Danish oil and others are really oil and varnish mixes.

 

General Finishes' Oil and Urethane Topcoat is not an oil and varnish mix but a wiping varnish.

 

Shellac is called padding lacquer when it has an oily solvent included for French polishing style touch-up.

 

Most finishes called friction polish are shellac and wax blends.

 

Water base finishes are called lacquer or varnish or whatever as a marketing ploy to make you think you're dealing with something familiar.

 

So Oil, Varnish, Shellac, Lacquer, water based – how do we sort this out? 

 

 

Types of Finishes

 

Finishes can be defined in terms of how they cure (reactive or evaporative) and whether or not they form a film on the surface of the wood. 

 


Reactive Finishes – Cure through a chemical process called cross-linking that changes the chemical structure of the oil or resin.  This process is accomplished by auto-oxidation or by means of a catalyst.  Once the finish cures, the solvent for the resin can no longer dissolve the cured finish.

 

Examples: oils, boiled or polymerized oil, varnish, 2-part finishes such as conversion varnish, catalyzed lacquer, and polyester.

 

Note:  Dense oily tropical hardwoods present 2 problems in finishing.  First, the oil can cause an adhesion problem.  Second, the oils in the wood contain anti-oxidant chemicals that retard/prevent auto-oxidation in reactive finishes.  This can be addressed with a seal coat of  shellac.

 

Evaporative Finishes – These cure through evaporation of the solvent. This does not change the chemical composition of the resin and the solvent will re-dissolve the finish.

 

Examples: shellac, nitrocellulose lacquer

 

Water- borne finishes, as opposed to solvent-based finishes, are a special case.  The finish is in the form of droplets suspended in water.  The water evaporates to initiate the cure, then the droplets ‘coalesce’ and cross link to complete the reaction.

 

 

 

Non-film Forming - oils and oil/varnish mixes are the only non-film forming finishes.

 

Film Forming - Everything else forms a film.

 

 

IV.     Rating Finish Types for Durability, Appearance, and Application

 

Durability

 

Film forming finishes are more durable than non-film finishes.  Reactive film finishes are more durable than evaporative film finishes.  The most durable are 2-part or catalyzed finishes.

 

Water based finishes are a special case of film forming finishes and in general are not as durable as their solvent based counterparts.

 

Appearance

 

Non-film forming finishes yield a natural 'close to the wood' appearance.  They normally darken the wood and tend to accent or ‘pop’ the grain, especially curly or bird's eye.  Film forming finishes yield a richer 'formal' appearance. Sheen can be controlled by additives or various rubbing techniques.

 

In general solvent-based products add some degree of amber color.  Water based products generally don't add any color.

 

Note: most finishes tend to yellow over time due to exposure to UV.  Some finishes will incorporate UV screens and absorbers to minimize this affect. However, acrylics (water or solvent base) are known as 'water white' and yellow little if at all over time. 

 

As a final note, the wood itself will change over time due to the UV (lighten, darken, and/or change color) depending on type of wood

 

Application

 

All application techniques generally require a well ventilated, dust free environment. 

 

Oils and wiping varnish are easy to apply but care must be taken to properly dispose of oily rages to avoid spontaneous combustion..

 

Spaying versus Brushing – Spraying is easy - brushing is hard.  Most spayed finishes dry tack-free quicker making dust less of a problem. Quick drying finishes (lacquer, shellac) can even be spayed outside in fair weather.  Most brushed finishes dry more slowly and as such are more prone to dust. In addition, bushing requires a practiced technique to avoid bush marks, bubbles in the surface film, runs, etc.  On the other hand, modern HVLP systems are affordable and make spraying easy and efficient.  And a fold-up spay booth can be fabricated for less than $100.

 

 

Application of Different Finish Types:

 

Oil and water based products can be brushed or sprayed and usually cure tack-free in less than an hour but still require a fairly clean environment. If brushing, care must be taken not to overwork the finish incorporating bubbles in the surface film.

 

Nitrocellulose lacquer and shellac can be brushed or sprayed, however spraying is preferred because of the fast dry time.  If brushing, flat artist brushes work best since they don't load as heavily as a regular brush.

 

Note:  Most of the above finishes are available in 12oz aerosol cans that are ideal for small projects. 

 

Shellac/wax or oil blends (friction polish and padding lacquer) are applied with a cloth and rubbed to a final cure.  This is a preferred finish for lathe work since it is applied with the lathe turning and the heat of friction speeds the cure.

 

2-Part or Catalyzed Finishes must be sprayed. Pre-catalyzed lacquer (catalyst is added by the manufacturer) is less durable than post-catalyzed (2-part) but is available in 12oz aerosol cans and has the same rapid dry time as regular lacquer.

 

V.      Review of Specific Finishes

 

Drying or Curing Oils 

-Marketed as Pure Tung oil and boiled linseed oil.

-Reactive non-film forming finish.

-Offer little protection.

-Used primarily to bring out or ‘pop’ grain features.

-Flood on, let soak in for 15 to 30 min., then wipe off excess.  Cure time is over night.

-Can be top-coated with a more durable finish.

-Yellows over time.

 

Myth:  Tung oil is an extremely tough and water resistant finish. 

Truth: Tung oil came into Europe at the time Europeans were using linseed oil as a finish and it was superior.  But with the advent of varnish it fell out of use as a finish.

 

Polymerized Oil 

-Marketed as Tru-oil or Gun Stock finish

-Reactive film forming finish

-Polymerized tung oil.  (tung oil heated in the absence of oxygen)

-Dries rapidly and builds up similar to shellac but is more durable.

 

Varnish

 

-Marketed as polyurethane, marine spare varnish, restoration varnish,

-Reactive film forming finish

-Natural or synthetic resin cooked with an oil then thinned.

-Solvent is turpentine or mineral spirits.

-Modern resins are usually alkyd, phenolic, or urethane

-Can be brushed or sprayed, available in 12oz aerosol cans.

-Long cure time (usually hrs to tack-free) requires a dust-free environment.

-Cures slowly but to a very durable, thick film.

-Most varnishes add a slight amber color and yellow over time.

 

 

Varnish/Oil Blends

 

-Marketed as Danish Oil, Tung Oil Finish, Antique Oil

-Reactive non-film forming finish.

-Oil causes the final finish to cure soft like a true oil finish.

-Cannot be built up to a thick film because of the oil.

-Applied the same as oil.

-Somewhat more durable than a straight oil finish.

 

Wiping Varnish

 

-Marketed as Watco Wipe-on Poly, GF Seal-a-Cell, GF Oil and Urethane.

-Not an oil/varnish mix but a true varnish thinned down with mineral spirits.

-Reactive film forming finish

-Multiple coats are required for good protection.

-Advantages - ease of application and shorter time to tack free.

 

 

Shellac

 

-Available as dry flakes in garnet, amber, and blond (de-waxed)

-Mix flakes with alcohol 1, 2, or 3lbs to a gallon of alcohol.

-Available pre-mixed as SealCoat (de-waxed), Amber and Clear.

-Available in 12oz aerosol cans.

-Evaporative film forming finish

-Dries tack-free in minutes

-Brush or spray.

-Beautiful film finish but not very durable.

-Excellent rubbing characteristics for high gloss finish.

-Yellows over time.

-Once mixed or premixed opened, it has a shelf life of 4 to 6 months.

 

-Also available mixed with oil or wax as friction polish or French polish.

-Apply with a rag using the heat of friction to accelerate the build-up/cure.

 

Nitrocellulose Lacquer

 

-Generally marketed as lacquer.

-Evaporative film forming finish.     

-Brush or spray – available in aerosol cans.

-Rapid dry time (tack free in less that 5 minutes)

-Moderate durability.

-Excellent rubbing characteristics for high gloss finish.

-Yellows over time

 


Pre-Catalyzed Nitrocellulose Lacquer

 

-Generally marketed as Pre-Cat Lacquer.

-Evaporative/reactive film forming finish.   

-Spray only but available in aerosol cans.

-Rapid dry time (tack free in less that 5 minutes)

-More durable than lacquer but not as durable as varnish.

-Excellent rubbing characteristics for high gloss finish.

-Yellows over time

 

Water-borne Finishes

 

-Usually marketed as water based something or other.

-Cure is a special case evaporative/coalescence process to form the film.

-Less solvents vapors.

-Resin may be about anything (polyurethane, acrylic, lacquer?).

-Doesn’t cross link as affectively as solvent-based, therefore not as durable.

-Brush or spray, available in 12oz aerosol cans.

-Doesn’t add color and little to no yellowing over time depending on resin.

-Dries faster than solvent-based but sensitive to temp and humidity.

-Because of the water, it raises the grain.

 

2 Part Finishes

 

-Marketed as post catalyzed lacquer, conversion varnish, polyester.

-Evaporative/reactive film forming finish.

-Very durable finish that rubs out well.

-Requires accurate mixing of the 2 parts.

-Produces harmful/flammable vapors requiring spray booth equipment.

-Does not yellow over time.

 

CA glue - The Finish that's not a Finish

 

-Cures to an extremely brittle, hard film.  Not applicable to large projects.

-Excellent rubbing characteristics for high gloss finish.

-Application similar to Friction Polish.

-Drizzle on as lathe turns slowly.

-Some recommend a drop of linseed oil on the rag to slow the cure.

-Sand to a high grit (micro mesh) to achieve the final finish.

-Excellent finish for small pieces that will see a lot of wear such as pens.

 

 


VI.   Summary

 

Wood is finished to provide protection and enhance its appearance.

 

Before applying a finish the piece is prepared by following an effective sanding protocol.

 

Selecting a finish involves considering how the piece will be used (durability), what we want the piece to look like (appearance), and finally what is involved in applying the finish (application technique).

 

Durability

 

Reactive finishes are more durable than evaporative finishes with the exception of oils.  Oils are non-film forming finishes that lack durability but are affective in enhancing appearance.

 

Appearance

 

Solvent-based finishes usually add some degree of amber color and yellow over time.  Water based finishes, solvent based acrylics, and  most 2-part finishes are ‘water clear’ and do not yellow over time.

 

Application Techniques

 

 

All application techniques require a well ventilated, dust free environment.  Oils and wiping varnish are wiped on with a cloth. Varnish, Shellac, water based, and Lacquer can be brushed or spayed.  2-Part finishes must be sprayed.  And in my opinion, modern HVLP systems have made spraying finishes easy and affordable.

 

Shellac, lacquer, and 2-part finishes dry very quickly and as such are not as vulnerable to dust during the cure.

 

Wiping oils and varnish produce oily rags that can spontaneously combust if not properly disposed of.

 

Spraying solvent based finishes produce toxic and potentially flammable vapors that must be taken into consideration.

 

 

 

 

 


VII.  Final Thought

 

Almost everything I know started with a book somewhere.  And what I know about finishing, in addition to that learned from hours of struggling with disappointment and success in my shop, came mostly from these three books:

 

Taunton's Complete Illustrated Guide to Finishing, Jeff Jewitt.  ISBN 1-56158-592-0

 

Understanding Wood Finishing, Bob Flexner.  ISBN 0-7621-0680-8

 

The New Wood Finishing Book, Michael Dresdner.   ISBN 9781561582990

 

I highly recommend them for anyone who wants to understand finishing from first principles as well as practical experience.