Safety First!
Safety in the workshop is YOUR RESPONSIBILITY alone; I make no warranty
as to the safety of any technique or tool shown or described on this site.
Before beginning any project, you must understand woodshop safety,
know how to safely operate any machinery that is to be used in the project,
and understand the safe use and any potential safety hazards involved in
the use of all materials to be used in the project. See the
general safety disclaimer and the
Chemical Safety Note for additional
information.
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A clear varnish is simply paint with no color. That's it, nothing fancy, just clear paint. Just like paint, a varnish is designed to encase the material it covers, protecting it from damage by abrasion, chemicals, ultraviolet light, and so on. Besides varnishes, there are also other clear finishes, such as oils and waxes, and all have their own unique properties which suit them for particular uses. Many finishes combine varnish resins, oils, and waxes in order to get the best properties of each, and minimize the drawbacks as compared to any one of them.
Moving into the details, finishes are grouped into two broad categories, based on what the finish does as it dries. Evaporative finishes harden simply by losing the solvent that kept them liquid when they were applied. If the solvent is applied to the dry finish, the evaporative finish will dissolve again, which makes repairs and refinishing very easy. Evaporative finishes include waxes, shellac, and nitrocellulose (NC) lacquer.
A reactive finish changes chemically as it cures (hardens). Once a reactive finish has cured, it can no longer be dissolved by the solvents that originally held it in suspension. This means that in order for subsequent coats of the finish to adhere, the surface needs to be deglossed, or scuffed, to provide for better mechanical adhesion between coats of the reactive finish. This also makes repairs and refinishing more difficult, but in general, reactive finishes are tougher than evaporative finishes. Reactive finishes include oil based varnishes, waterborne varnishes, and drying oils such as tung oil, boiled linseed oil, and others.
Speaking of varnishes, this would be an opportune time to clear up a bit of confusion about finish names. Simply put, a varnish is any film-forming finish; it seals the wood under a layer of natural or plastic resin, protecting it against scratches and stains. Varnishes include polyurethane, as well as shellac and lacquer. A non-film-forming finish, such as boiled linseed oil, tends to soak into the wood and harden inside the wood. Because the finish is in, rather than on top of the wood, a penetrating oil finish cannot protect the wood as well as a film-forming finish.
Clear Finish Selection Key
Step One: item location. Where will the finished item be located?
Outdoors - 1
Indoors - 2
1. Outdoors:
The main enemies of wood outdoors are water and ultraviolet. Dry wood tends to repel insect and fungal attack, so sealing out water is doubly important. The next question involves the relative importance of fade resistance, ease of maintenance, and overall durability.
Some fading & weathering, but very little work - 1.1.
Colorfast with great durability & less work - 1.2.
Colorfast with maximum durability & regular maintenance - 1.3.
1.1. Outdoor Oil finishes are oils that contain U.V. blockers and fungicides, so they are specially designed to mostly seal the wood and protect it from sun, insects, water, and rot. Since the finish is in the wood rather then on top of it, the surface is exposed to much more weathering than with a varnish. For staining, a pigment based stain is the only real choice for outdoor projects. Dyes bleach much too easily in direct sunlight. Waxes are too thin and are much too sensitive to heat, so they are also useless outdoors.
Continue with 3.
1.2. Exterior and Marine or Spar Varnishes give more protection for a clear finish, but at the cost of more tedious maintenance. (Paint offers the most protection of any wood finish, since it is generally thick and also opaque.) Exterior varnishes contain U.V. blockers to maintain the wood's color better, and since they form a film over the wood, they keep the wood drier, thus preventing warping, broken joints, and decay from bugs and rot. Spar varnishes do tend to have a strong yellowish cast, but there are some products that remain fairly colorless. For staining, a pigment based stain is the only real choice for outdoor projects. Dyes bleach much too easily in direct sunlight. Waxes are too thin and are much too sensitive to heat, so they are also useless outdoors.
Continue with 3.
1.3. Composite Finish (fiberglass, epoxy, & varnish)
Tx
Continue with 2.3.6.
2. Indoors:
The next question for an indoor project relates to whether the wood is open or close grained. Softwoods like pine and redwood, and hardwoods like maple and cherry are all close grained. Open grained woods, like oak, pecan and mahogany, have large pores on their surface that contribute to their characteristic texture.
Open grained woods, go to 2.1.
Close grained woods, go to 2.2.
2.1. Open grained woods are sometimes treated with a grain filler to fill up the open pores on the wood's surface, allowing them to take a glassy smooth finish. This adds extra time and work to the finishing process, but gives a more formal finish that is also easier to keep clean.
For an open-pore finish, continue - 2.2
For a filled-pore finish:
If a filler is to be used, the wood needs to be dyed first (if desired, see 2.2.1), then sealed with a thin sealer. Once the sealer has dried, the grain filler can be applied, allowed to dry, and carefully sanded. Stain comes after grain filler, and since most fillers do not accept stain, it is best to choose a filler that is a slightly darker color from the stain, so it will match color and still highlight the grain pattern of the wood.
2.2. Staining:
Wood can be colored by applying a pigment stain (like very thin paint), a dye (chemical coloring), a glaze (a stain applied between coats of varnish), or by a combination of these. Each method has its advantages and disadvantages. When used in combination, these techniques can produce very transparent, rich colors that cannot be obtained through the use of any single coloring agent.
Stains:
A stain settles into any crevice in the wood's surface, so it highlights
the grain patterns in oak, but it also highlights any coarse sanding scratches
as well, and it may blotch on some woods due to uneven porosity of the wood's
surface. A stain will also tend to be more fade-resistant.
Dyes:
A dye colors the wood on the molecular level, so it will leave the pores
on oak the same color as the rest of the wood, thus minimizing its grain
patterns. Dyes work best to highlight the ripple patterns of figured maple
without going blotchy due to the irregular grain, the way a stain does.
Dyes also excel at producing bright primary colors on wood, although
they are not as fade-resistant as pigment stains.
Glazes:
A glaze is simply color applied over sealed wood. A glaze can help
to bring out more texture than dye alone, especially on oak, but even on
maple, without the tendency to blotch. Glazes are also useful for special
effects such as antiquing.
Toners and Tinted Finishes:
Both of these are used in a similar manner, just for slightly different reasons.
A toner is like a stain that is meant to be lightly misted onto the
surface to fine-tune the stain color for an exact match between a new piece
and an existing one. A tinted finish is a clear finish that has stain
or dye added to it to give it a slight tint. It is also used to adjust
the color of the finish, but is generally not used for such exacting jobs
as matching existing colors. Tinted finishes tend to produce richer
color and greater depth than toners.
For very even color, especially on fine-grained or blotch prone woods, use dyes - 2.2.1.
For color that highlights the pore structure of woods like oak, use pigment stains - 2.2.2.
For more control on difficult woods, to highlight texture, or for special effects such as antiquing, use glazes - 2.2.3.
For fine adjustments to the color, use a toner or tinted finish - 2.2.4
To let the wood's natural color shine through, use a natural (no stain) finish - 2.3.
2.2.1. Dyes, also called aniline dyes, are very easy to use and also fairly forgiving, but they do have their own quirks. >>Sealing<<
Continue or skip to 2.3
Dying technique…
Seal the wood, then:
Continue with a glaze - 2.2.3
Adjust with a Toned or Tinted Finish - 2.2.4
Skip to Clearcoat - 2.3
2.2.2. Pigment Stains may require the use of a sealer or a conditioner to allow the stain to evenly color the wood without blotching. Blotch prone woods include poplar, birch plywood, pine, and cherry. It is always best to check a sample of any wood for blotching prior to staining, and it doesn't hurt to take precautions anyway, since once the stain hits the wood, there's no going back.
Staining technique…
Continue with a glaze - 2.2.3
Adjust with a Toned or Tinted Finish - 2.2.4
Skip to Clearcoat - 2.3
2.2.3. Glazes are a technique for applying color as well as actual finishing products. The idea is to apply color over a sealed wood surface, so the color is more or less independent of the wood's grain and fine texture (thus, no blotches). A glaze can be a gel-like stain, wiped on and wiped off, so it stays in crevices and gives an "antique" look. Another glazing technique involves using a tinted finish, usually sprayed on in several coats, to achieve subtle color gradations over the workpiece (such as a sunburst effect), or an even adjustment to the project's color, or some other special effect.
Adjust with a Toned or Tinted Finish - 2.2.4
Continue with Clearcoat - 2.3
2.2.4. Tinted Finishes and Toners…
Continue with Clearcoat - 2.3
2.3. Topcoats
Selection brief
2.3.1. Wax
2.3.2. Oil
2.3.3. Shellac / Lacquer
2.3.3.1. French Polishing
2.3.3.2. Brushing
2.3.3.3. Spraying
2.3.4. Danish Oil
2.3.5. Waterborne Varnish / Lacquer
2.3.5.1. Brushing
2.3.5.2. Spraying
2.3.6. Oil Base Varnish
2.3.6.1. Brushing
2.3.6.2. Spraying
2.3.7. Pre-Cat / Post-Cat Varnish / Lacquer (cross linked finish)
2.4. ??
3. Finishing the Finish
3.1. Rubbing Out / Deglossing
3.2. Wax
References:
Title, Author, Publisher, Date