Anyone that lived through the 1980s, especially South of the Mason-Dixon, has probably seen their share of red oak cabinets and furniture. Red oak is plentiful and cheap in the southeastern US. It is strong, but works easily with sharp tools. It even carves relatively well.
But red oak has developed somewhat of a reputation for being a ‘plebian’ material, not suitable for fine furniture.
While I agree that red oak has a certain coarseness of appearance, I see no reason for that characteristic to prevent its use. When the texture is more than desired, a old woodworker’s trick can make the humble red oak as smooth and silky as any maple, cherry or walnut. And with some attention to material choice, it can have the appearance of an exotic hardwood.
What I’m talking about is called “filling the grain“. There are many ways to do it, but in every case, the purpose is to fill in the open pores of the wood, allowing the creation of a very smooth surface.
There are many commercial products called “grain fillers”. These should NOT be confused with products called “wood filler”. Wood filler is meant to patch holes or repair damage. Grain fillers are only meant to fill the pores of the grain to create a level surface.
Commercial products like Timbermate or Aquacoat are readily available, but can be costly to use on large pieces. Most builders will use such products only on smaller pieces, or just the “show” surfaces of larger pieces, like the top of a writing desk.
Also, the commercial products generally come in one of two forms: pre-colored to “match” the wood species desired, or clear. In my experience, the color match is difficult to hit exactly, and clear can create a somewhat unnatural appearance, as one can see through to the bottom of the pores. Either of these types of product can certainly be used to good effect, but I have a fondness for using old-time methods.
Plus, I’m cheap.
When filling grain in a species with relatively small pores, walnut for example, the shellac technique works well. This procedure has one apply a coat of shellac, and once dry, scrape or sand back to the wood surface. After a few repetitions, the grain pores will be filled with whatever color shellac you choose, and the final top coats can be applied. This is particularly effective when creating a “french polish” finish. However, it can be very tedious, and not inexpensive, to use on large pieces, especially with open-pored wood species.
A variation on this technique uses an oil-based finish, rather than shellac. Using a fine abrasive paper, lubricated with a dilute version of the oil or varnish that will used as a top coat, one should sand the surface to create a slurry of wood dust and finish. Working this slurry into the grain, then wiping the excess away after a few minutes, leaves the pores filled, and perfectly matched in color. Note that this is NOT a good choice for inlays or pieces with mixed colors.
This brings me to the subject of this article: Plaster of Paris as a grain filler.
Plaster of Paris, also known as gypsum or calcium sulfate hemihydrate, is a fine white powder. Known and used since ancient times, this plaster has characteristics that make it a good choice for filling grain in open pored woods.
The crystaline structure of the powdered particles offers sharp edges that embed into the wood fibers, helping to lock the material in place. When mixed with water, this same structure helps the particles to interlock as as the mixture begins to cure, causing the mix to solidify long before it actually dries out. Plaster adds bulk much faster than slurry from wet sanding will build, so it fills the large, open pores of red oak much faster.
Plaster is also somewhat porus when set and dried. This allows it to absorb oil finishes and turn relatively* translucent, rather than keeping its chalky white appearance.
*The porosity of dried plaster does allow it to absorb things like boiled linseed oil, but oil-based stains tend to sit on the surface. The result is chalky white, so consider this when planning your finishing schedule.
Another advantage of plaster is that it can be tinted to mute, or to highlight, the grain it fills.
Here is the technique I use for oak:
- Sand the surface to at least 180 grit, and clean it thoroughly.
- Mix powdered plaster and water with water-soluble black dye, to form a medium paste. Small batches are best.
- Spread the paste onto the surface with a spreader, taping knife or trowel. Work quickly to push the paste into the grain, plaster sets in minutes.
- Allow to dry, normally at least two hours.
- Use a card scraper to remove the bulk of the plaster from the surface. This produces far less airborne dust than sanding.
- Use a random orbit sander attached to a vacuum to clean the surface, leaving only the plaster embedded in the grain pores. I usually start with 80 grit.
- Work up through the grits, scribbling pencil marks on the surface before each grit and removing them with the sander. This ensures that you sand the surface completely with each grit.
- Oak is usually good to stop at 180 grit. At this point, you may decide that another application of plaster is needed, if you want a perfectly smooth surface texture. Normally, I stop here and leave just a touch of grain to feel. Repeat the application and removal as desired.
- With colored plaster, you can proceed directly to stain or topcoat. With natural white plaster, a coat of boiled linseed oil, thinned with spirits, and warmed if the room is cold, will penetrate the plaster and make it translucent. Once the oil cures, move on to the rest of your chosen finishing schedule.
One final tip: when removing the sanding dust prior to application of stain or top coat, do not use compressed air or vacuum. It is possible to dislodge some of the plaster with these methods. Instead, use a tack cloth or a rag moistened with mineral spirits to remove the dust.
Another useful technique, which I learned from studying the writings of a piano builder, is to “fad” the plaster into the grain with a cotton pad, similar to that used for French Polishing shellac. This is the technique described for creating the glossy-smooth surfaces on a grand piano.
- First, make the “fad” by wrapping a lint-free cotton cloth around a small wad of cotton cloth, cotton balls, or wool. Tie the cloth to form a firm ball-shaped pad, with a “pony tail” to help grip it.
- Dip the fad into clear water to saturate it, then squeeze out the excess.
- Dip the moistened fad into the dry plaster powder, enough to pick up a small amount.
- Rub the powder into the wood surface with the moist fad. Use small circles, similar to waxing a car. The moisture in the fad will mix with the plaster to form a paste.
- Use the fad to push this paste deep into the wood pores, until they are completely filled. Repeat the process, adding water and powder as needed, until the entire wood surface is covered.
This “fadding” process has the advantage of creating less build-up to remove from the wood surface. It is also easier to cover a large area, since the plaster and water are mixed during the application. Pre-mixed plaster requires fast action, as it sets rather quickly. Coloring the plaster while fadding doesn’t work so well, but dye can be added to the surface afterward, if desired.
Here is a short demonstration of how I apply plaster as a grain filler:
Until next time –
Savor the Sawdust!