All hardwoods have vessels in their cellular structure for the conduction of sap when the tree is living. When the wood is cut into lumber, these vessels are cut, much like a straw cut at an angle. These cut vessels are called pores and the size and distribution of these pores are what give hardwoods their individual character. Pores can be small and hard to see as in like cherry, maple and poplar. Other woods have large pores, like ash, oak, mahogany and walnut.
What you do with the pores during the finishing process largely determines the overall aesthetics of the finished piece. Applying a stain and finish to woods with large pores leaves the pores open and this is called an open pored finish. The pore depth is still visible when the wood is viewed at certain angles. (Drawing A.). When the pores are filled -- either with finish or a product called paste-wood filler -- the wood surface is completely smooth and flat when viewed at any angle and this is called a filled pore finish or a "piano" finish. When the pores are filled and the finish is buffed to a high gloss, a mirror-like surface results and this finish is seen on high-end furniture and musical instruments like guitars and pianos.
Producing a filled pore finish by filling the pores with paste wood filler takes a little practice and preparation - but the results can be stunning. The traditional paste wood filler is oil-based but water-based formulations are now available which have some distinct advantages. In this article I'll discuss the different paste-wood fillers and how to apply them.
WHAT PASTE WOOD FILLERS ARE
All paste wood fillers, whether oil or water based, are based on the general formulation of bulking agent, carrier, and binder.
Bulking agent - In all paste wood fillers the bulking agent is the material that fills the pores. This needs to be an inert substance with minimal shrinkage. Very finely ground silica, called silex, is the bulking agent most widely used.
Binder - Something needs to hold the bulking agent to itself and in the pores. Binder is a finish resin, either oil/varnish or acrylic.
Carrier - This is the solvent compatible with the resin. Mineral spirits is used with the oil/varnish and water with the acrylic resin.
Paste wood fillers are available in neutral, which is the natural color of the materials used. Colored paste wood fillers are available which have a colored pigment ground into the mixture which can blend in with the color of the wood or add a contrasting effect. Many shops purchase neutral and tint the filler with dry pigments or a tinting medium compatible with the filler. Water-based fillers can be tinted with dry pigments (sometimes called fresco powders), universal tinting colors, and artist acrylic colors. Oil-based products can be tinted with dry pigments, universal tinting colors, japan colors, and artists oil colors. (See photo of board with various tinting mediums)
The oil-based formulations have been around the longest. They are easy to apply but require a long drying time - anywhere from 2 days to a week in extreme temperatures. They are compatible with most finishes but can cause problems with water-based finishes. Under oil-based varnishes they can cause problems if not completely dry. The water-based formulations are newer and have the advantage of being compatible with all finishes. In addition, they dry much faster - most water based fillers can be topcoated after 3-4 hours drying time. They are a little trickier to use- but in my shop the shorter drying time and universal compatibility is a big plus.
Whichever filler you use, surface preparation is important. The wood should be carefully sanded to at least 220 grit and the sawdust removed from the pores. Use a compressor or a vacuum. Small pored woods like cherry, maple or poplar generally do not need a paste wood filler. After sanding and removing the sawdust, wipe the wood with naphtha to see if there are any blemishes on the wood that you may have missed. Colored paste wood fillers will collect in large sanding scratches and other surface defects so careful attention to surface preparation is important.
Apply paste fillers to either the raw wood or after the wood is stained and sealed with a coat of thinned finish. If you apply the filler directly to the raw wood, use a neutral filler if you want a natural color to the wood. Or you can use a tinted filler which stains the wood and fill the pores at the same time. Since paste wood fillers seal the surface of the wood when fully dry, staining the wood after application is difficult. (An exception is water-based fillers as discussed below). I recommend staining with a dye-type water stain. If you should happen to sand through the stain later, it's much easier to touch up the wood with re-application of the dye. Most professional finishers apply a sealer coat of finish before application of the filler. This gives much greater control over the final color of the piece and also allows for certain creative effects. One effect that I like is to leave the color of the wood natural and use a dark paste wood filler in the pores. Conversely, you could use a lighter colored filler which can result in very striking contrasts. (Photo, also possibly a chart.)
Either filler is applied in generally the same way - the filled is applied to the surface, packed into the pores and the excess removed. The technique is different for oil-based and water based so I will discuss each one separately.
USING OIL-BASED FILLERS
Oil-based fillers are straightforward to apply, but are not without problems. The most common are waiting too long for the filler to dry before removing, not letting the filler dry long enough, and filler marks under the finish. Following the procedure outlined below should prevent most problems.
Oil-based fillers should be thinned to the consistency of thick cream. Manufacturers usually tell you right on the can whether or not the filler needs to be thinned. Thin fillers are easier to apply and clean off than stiff fillers. Use naphtha to thin the filler, paint thinner or mineral spirits will extend the drying time.
Apply the filler with a stiff bristle brush. I like to use an inexpensive polyester brush with blunt edges or an old worn out black bristle brush. Apply the filler liberally to the surface in any direction working it into the pores of the wood with the tip of the brush. It's best to work with small, manageable areas on large surfaces, until you get the feel for how fast the filler dries. Immediately after brushing, take a rubber squeegee (available from source of supply) or a thin scrap piece of wood and scrape off the excess filler at a 45 degree angle to the grain of the wood. This step is important, because it packs the filler into the pores. After scraping the excess, wait for the filler to haze over. This can be anywhere from five minutes to 20 minutes, depending on temperature and humidity. As soon as the filler hazes, take a piece of burlap, about 12 square inches, and start rubbing off the excess filler from the surface. Start by rubbing across the grain. (Photo) Rubbing with the grain pulls the filler out of the pores. Switch to a clean area of burlap often and continue rubbing until no filler shows on the burlap. Then switch to a clean area of burlap and lightly rub the surface in a figure-eight pattern. This removes and evens out any cross-grain lap marks from the filler. Removal of cross-grained streaks is important because they will be very visible once the finish is applied. Any filler that has dried too long and is hard to remove can be removed by squirting a small amount of naphtha on the burlap -- this re-dissolves the varnish binder.
Let the filler dry at least 12 hours and then examine the surface. Large pored woods like oak may need a second application of filler, while smaller pored woods like walnut usually only need one application. Apply the filler as described above if a second application is needed.
Many problems are caused by not letting the filler dry long enough. Two days is a minimum. In humid weather or if your shop is cold, wait at least a week. After the filler is dry, lightly sand the wood in the direction of the grain with 320 grit stearated sandpaper. (Sandpaper with zinc stearate added to minimize clogging). (Photo) Go lightly to avoid cutting though edges or to the bare wood. If you go through the stain and you used a water-based dye-type, simply re-apply more dye with a small brush. The sand-through will be invisible. I do not use a sanding block. I fold the paper into thirds and back it up with the palm of my hand. A problem that finishers run into at this stage is that they think the filler hasn't dried because it gums up the sandpaper. This happens because the fillers are formulated with linseed oil to aid in the application and scraping off. Linseed oil does not dry hard like varnish, it's much softer and flexible so it gums up the paper.
USING WATER-BASED FILLERS
The procedure for applying water-based filler is the same as with oil-based fillers but with several variations. Water-based fillers dry very fast and if the filler dries, you can't remove it except by sanding. The best way to use water-based is to apply it and scrape it off immediately. I use a rubber squeegee like the one in the photo. I brush a liberal coat of filler on and then immediately remove the excess with the squeegee. Do not wait for water-based fillers to haze. After scraping, wipe the board with burlap, cross grain at first, and then switching to figure-eights. You probably won't be able to remove all the filler, but that's fine, you'll remove it with the next step. Wait at least an hour and then sand the dried filler off with 240 or 320 grit stearated sandpaper. If the paper gums, the filler hasn't dried fully -- wait another hour and then try again. The filler should powder easily like in the photo. Applying a sealer coat of finish really helps in the removal of the water-based filler, but nonetheless, speed is important.
Water-based fillers differ primarily from oil-based fillers because they take certain stains after application. This means that you can use a neutral filler on raw wood, remove the bulk of it and sand it flush to the surface of the wood after an hour. You can then apply a stain (alcohol dyes and NGR stains are best) and this will color the wood and the filler. Add alcohol to water dyes. If you use a straight water based stain, it will color only the wood. Wait at least three hours (but no longer than 12 hours) to apply the stain. Water-based fillers are dry enough to topcoat with finish after three hours. Any finish can be used; water-based, shellac, lacquer, varnish or polyurethane.
I've used oil-based fillers since I started finishing and the advantages are ease of application and wide availability. The disadvantages are long drying time, limited finish compatibility, and some finish problems. The most common problem is due to not letting the filler dry long enough and this can result in white or grayish spots in the pores under the finish topcoats. If this happens, you have to strip the finish off and start again. Unfortunately, this problem usually doesn't appear right away, it may be months before it's noticeable. Most professional finishers "lock" in the oil fillers with a wash coat of shellac. The shellac acts as a barrier between the filler and the solvents in lacquer and varnish which can cause the filler to soften or swell.
If you've never used paste wood fillers, I'd recommend the water based filler and suggest practicing on lots of scraps using the techniques outlined above. Try using different colored fillers on raw wood. Also, try sealing the wood first then applying different colored fillers for various effects.
I've been using water-based fillers more and more. They aren't harder to apply, you just have to adjust your technique if you're used to working with the oil-based fillers. The quick dry time is a big advantage in production situations like my shop and the ability to stain after the filler is applied allows more control over color. They also sand much easier. Their universal finish compatibility allows me to use the water-based product under all finishes. Also, they are non-flammable, a feature my insurance company likes. The only disadvantage is that they'll freeze - so keep them stored in a warm place and if you order them by mail, try to order in warmer months.
Water, oil based fillers, rubber squeegee applicators, fresco powders, japan colors are available from:
H. Behlen and Bros.
4715 St. Hwy. 30
Amsterdam, NY 12010-9204
518-843-1380 Call for the name of a dealer near you.
Water-based fillers available from:
1045 N. Highland Ave.
Atlanta GA 30306 404- 872- 4466