The Finishing Touch
A Guide to Specifying North American Hardwood Species & Finishes
Tips & Techniques

THE BASICS OF HARDWOOD FINISHING

Whether you finish on-site or choose a factory application, it’s helpful to understand the basics of hardwood finishing. Finishes enhance or alter the natural beauty of wood, and protect wood from damage.

Wood Preparation

The most important step in hardwood finishing is wood preparation. Indentations, grease, oil and other wood defects are removed with multiple grits of sandpaper, scraping, and even sponging and then sanding. The stain and finish then can be taken into the pores evenly across the whole surface.

The cellular structure of a wood influences its appearance. Open grain hardwoods, such as elm, oak, and ash are “ring-porous” species. These species have distinct figure and grain patterns. Close Grain hardwoods, such as cherry, maple, birch, and yellow poplar, are “diffuse-porous” species. Most North American diffuse-porous woods have small, dense pores resulting in less distinct figure and grain.

Certain closed-grain woods such as cherry and maple tend to develop finishing blotches caused by textured grain. No matter how much sanding is used on these areas, they will never disappear.

A stain’s penetration also depends on the final grit with which the wood was sanded.. For instance, the large valleys left by 80 grit sandpaper will absorb much more pigment than will the fine tracks left by 150 grit sandpaper. For this reason it is important to prepare samples with the actual grits and the actual stains and finishes.

Colorants

After the wood has been well-prepared it is ready for even absorption (Into the wood) or absorption (onto the wood’s surface) of dyes, stains or fillers. In many cases, it is appropriate to sponge the surface with water, alcohol or solvents to show any areas which may contain glue marks, oil or uneven sanding. Colorants can be broken into two large groups: dyes and pigmented stains.

Budget

Pre-finished hardwoods can cost as much as $1.50 per linear foot more than unfinished products. However, there are several ways to look at the “real cost” question. While pre-finished material cost more initially, builders can save during installation. Finishing a hardwood floor to factory standards in the field can tie up a worksite for five days; even finishing decorative millwork can take three days or more. You’ll need a crew of finishers and extensive site preparation. Other work that could jeopardize the quality of the finish will have to top, adding time to the construction schedule.

If you finish on-site

Most builders still consider finishing hardwoods on-site the most practical alternative, especially for major projects like flooring, staircases and paneling. Attaining a high quality finish on-site is possible, but it requires planning, time and careful attention to detail. The deep, glossy, pre-finished look takes more than the standard three steps. The keys are repeated sanding and multiple finish coats.

1. Let the wood adjust to the site. Unfinished wood needs protection from moisture and from extreme fluctuations in temperature and humidity. Before the materials are delivered, have all windows and exterior doors in place, and brings the house to occupancy level conditions. Make sure all new drywall and plaster have had at least a week to dry. Store the unfinished wood in the room where it will be dimensionally stable when the time comes for installation and finishing.
2. Seal it. Solid hardwoods must be sealed on all sides since moisture can pass through the back, edges or ends as easily as the exposed faces. To avoid shrinking, swelling and other damages, you should pre-coat surfaces that you won’t be able to reach after installation.
3. Smooth it. Sanding is critical to an attractive, durable finish. Items like paneling, stair rails or cabinetry usually require only light sanding. Hardwood flooring needs to be sanded at least three time, with successively finer grades of paper. Vacuum the floor thoroughly after each sanding, and delay spot filling and other minor repairs until just before the final sanding pass.
4. Prepare the room. Once you’ve installed and sanded your work, the goal is to re-create, as nearly as possible, the finishing plant’s environment. Keep dust to a minimum and maintain stable levels of temperature and humidity. Seal doorways with plastic film and schedule other workers away from the jobsite. Mask off any surfaces you wish to protect.
5. Clean the wood. Before applying stain, wipe it down with a clean cloth dampened in mineral spirits to temporarily emphasize defects such as sanding marks. Smooth out any flaws that appear, then remove all dust with a brush, compressed air or a tack cloth.
6. Stain it. Apply stain generously with a brush, rag or lambswool applicator. Allow it to sink in for five to ten minutes, then wipe off the excess with a clean rag. Let the stain dry 24 to 48 hours – follow the manufacturer’s instructions.
7. Seal and finish coats. When the stain is dry, brush on the first finish coat and let it dry according to the manufacturer’s directions. Drying or curing times vary depending on the type of finish you use. When it’s bone-dry, sand the surface lightly with 280-grit or finer paper. Remove the dust with compressed air or a tack cloth and brush on the second coat.
8. Sand and coat again. The second of third finish coat will usually give you the desired result.

Dyes

Dyes are a solution of colorants in mineral spirits, oil, alcohol or water. Modern woodworking dyes are very similar to those used by dyeing cloth, paper or leather. Dyes change the hue of the wood without hiding its figure or rendering the surface unclear or muddy. In addition, dyes penetrate both the soft and hard grain, and often enhance or “bring out the grain”.

The molecular size of dye particles is much smaller than that of mineral particles in stains, and thus dyes appear more transparent. In addition, dyes bind to the wood because of their chemical molecualr structures and no additional binder is required. With water-based dyes, as the water evaporates, the wood is left with only the color in its surface.

The four main types of dyes are: water, alcohol, oil and solvent-based. Each has a particular use in the coloring process. For instance, with maple or birdseye maple, it may be found that water-soluble dyes penetrate more evenly across the whole surface. On a more oily wood, oil- or solvent -based dyes may penetrate better.

Oil- and solvent-based dyes may be added to pigments stains to form a combined stain and dye. This process may be able to yield the same color with only one operation.

Dyes are less light-fast than pigmented stains, and in renovation work this can be used to an advantage.

Stains

Produced from a variety of synthetic and organic mineral sources, stains are dispersions of particles of pigment within an oil or solvent base. The pigments are ground into fine particles and come in a powdered form which maybe added to other stains to increase color and density,

The depth of a stain and its opaqueness are directly related to the pigment’s solubility, as well as the size and density of particles in the solution.

Stains may be sprayed, brushed or wiped onto the surface. The depth and final color also are determined by the length of time the stain is allowed to remain on the surface and how hard the stain is wiped off.

When using very dark colors, it is difficult to reach the saturation of color without also covering the grain. For this reason, many projects are colored with a dye and then stained to produce a clear but dark color.

Pigmented stains will fill the grain and leave wood surfaces with less colorant, whereas dyes will stain the grain, and the areas between the grain, approximately the same color.

Wood Fillers

Woods with open grain structure such as oak and ash have very large pores. In some cases it may be desirable to fill the pores to allow the surface to appear smoother or the pores to appear lighter. Oil-based fillers may be mixed with oil-based stains to even the wood’s color and fill, in one step. Fillers are applied in a heavy film or paste form either by spraying or brushing, and the excess is wiped off with rags, scrapers or burlap, flush with the top of the wood surface.

Sealers

All finishes require a sealer. The finish can be self-sealing or the sealer can be different from the top coat.

In many cases, a vinyl sealer is used to lock in the color and protect the grain. Vinyl sealers even are used under oil-based finishes. The sealer locks in the colorants, seals the grain, starts the filling process, and provides a sandable coating. In many cases, the sealer is filled with particles which make the surface easier to sand but can also inhibit clarity.

Glazes

Glazes are transparent or semi-transparent stains which are applied after the sealer coat. In some cases, a transparent alcohol dye may be applied as a glaze as well. Glazes are often used to even out a light and dark area after the staining and sealing process. It is only after the sealer has been applied and sanded that the true color of the wood can be seen. This also is typically the time when touch-up of glue marks and unevenness of color can be corrected.

Toners

Toners are tinted applications of sealer or top coat which are used to build up a color. Toners tend to cover the grain because they are essentially pigments suspended in the finsih.

Check the Moisture Content

Moisture content is the amount of moisture is wood, expressed as a percentage of its oven-dry weight. The proper moisture content for kiln-dried hardwood products varies in different regions of the country. When receiving a shipment of hardwood paneling, flooring, millwork or cabinetry, pull out several sample boards and use a hand-held moisture meter to measure their moisture content. The figure should closely mach the one recommended for your part of the country, It it doesn’t take the following precautions.

Let Hardwoods Adjust to Their Surroundings

Sawmills dry and condition hardwood lumber to the moisture content expected in service. During transport and storage, hardwoods can accumulate moisture or become too dry. To avoid problems, let hardwood products adjust to the site before installation. Here’s how:

• Have Hardwood products delivered after all windows and doors are installed, and masonry and plastering are complete.
• With the home’s HVAC system operating at a normal level, unbundle and store hardwood paneling, flooring and millwork for three to five days in the room where it will be installed. If delivered before the HVAC system is operational, store the material in a dry environment until you can follow the instructions above.

Plan for Natural Expansion and Contraction

Here are a few important installation tips for minimizing expansion and contraction of hardwood paneling, flooring and millwork during seasonal changes in relative humidity:

• Install hardwood paneling so it moves across the grain. Use narrow widths when possible.
• For large areas, such as paneling, leave an expansion gap to accommodate the natural expansion and contraction, based on changes in humidity specific to your region. Conceal the edge with baseboards or mouldings.
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