Oil-based polyurethane is a very durable and hard-curing finish. It bonds well to itself, especially if each coat is sanded a little after it has dried well enough so it powders. This creates fine scratches, which improve the bonding of the next coat.
It’s a good idea to do this fine sanding between coats anyway to remove dust nibs.
But polyurethane doesn’t bond so well over finishes marketed as sealers, especially over sanding sealer. This sealer is good for use under non-polyurethane varnishes because regular alkyd varnishes gum up sandpaper. So to speed production, a sanding sealer can be used for the first coat. Sanding the first coat not only removes dust nibs. It also removes the roughness caused by the swelling of the wood fibers.
Shellac can also be used to seal wood under polyurethane. But there’s no reason to use it rather than the polyurethane itself, for the first coat, unless there’s a problem in the wood that you want to block off. Problems include pine knots, silicone from furniture polishes (which causes “fish eye” or “cratering,” especially on old wood that is being refinished), and odors from smoke damage or animal urine. For these cases, applying a first coat of shellac usually blocks off the problem.
If you do use shellac, you should use the dewaxed variety. The commercial product, available in home centers and paint stores, is SealCoat. Or you can buy dewaxed shellac flakes and dissolve your own in denatured alcohol.
The resin in pine knots contains solvents that will bleed into and through most paints and finishes. This can cause the paint or finish to remain sticky, and it can cause the orange color to bleed through as shown in the accompanying picture of white latex paint applied over pine.
There are two types of products on the market that will block this resin: white pigmented primers and clear shellac. The most well known white primers are Kilz and BIN. The best clear shellac to use is Zinsser SealCoat because it has very little color and a longer shelf life than other shellacs.
Kilz and other brands are available as oil-based and water-based. In comparative tests I’ve found that oil-based works better at blocking the resin than water-based. But the most effective blocker is shellac, both clear (SealCoat) and white-pigmented BIN.
Once you’ve applied the SealCoat or BIN to the wood, you can apply any paint or finish over it. As long as you haven’t sanded through, this first coat should block the resin.
Finishes differ in the amount of color they add to wood. Though you may not notice much of a difference if you are applying the finish over a stain, there is a significant difference when no stain or other coloring steps are used.
In the accompanying picture, you can see the differences clearly.
On the far left is paste wax. It adds almost no coloring to the walnut. Next is water-based finish, which also doesn’t add color, but it does darken the wood a little because of the penetration.
In the middle is nitrocellulose lacquer, which adds a slight yellowing to the wood. Clear (bleached) shellac is very similar. Next is polyurethane varnish. Varnishes can differ quite significantly in the amount of yellow/orange color they add to the wood. But most polyurethane varnishes are very close to this.
Finally, on the far right is orange (amber) shellac, which adds more orange coloring than any other finish.
So a big consideration when choosing a finish for unstained wood is the color you want to add. One of the main reasons, in fact, for choosing a water-based finish is to keep the color of the wood the same while still achieving a protective coat (which wax doesn’t provide).
The usual finishes that are sprayed are lacquer, shellac and water-based finish. These finishes dry fast, sometimes too fast in warm temperatures to successfully brush onto large surfaces. Spraying overcomes this problem.
Oil-based varnish, including polyurethane varnish and oil paint, can also be sprayed, of course, because any thin liquid can be sprayed. But you need to be aware of a significant difference. Varnish dries much slower than the other finishes, and unlike lacquer, shellac and water-based finish, each coat should be allowed to fully dry before the next coat is sprayed.
You may typically spray lacquer, for example, spray several coats at a time without allowing drying in between. The drying will be a little slower because there is more solvent in the thicker application that has to evaporate for the finish to harden. But the time difference won’t be great, and there won’t be any problems unless you build the finish so rapidly that it sags or puddles.
Varnish and polyurethane varnish (and oil paint) are different. For varnish to cure, oxygen has to work its way into the finish to cause it to crosslink. If you spray several coats of varnish one after another, the top surface will dry well before the varnish at the bottom, and this will significantly slow the penetration of the oxygen. The result will be a finish that feels dry at the surface but is still so soft that you can indent it with your fingernail, and it will remain like this for many days or weeks.
You can always allow way more time for the finish to cure all the way through. You can also speed up the curing by raising the heat in the room or applying heat with a heat lamp.
But this could cause the finish at the surface to wrinkle as the finish at the bottom dries and shrinks.
So it’s best to allow each sprayed coat of varnish or oil paint to fully dry (enough so sanding produces dust) before applying the next coat.
To achieve an even coloring with darker colored woods, it’s always best to use only heartwood to begin with. But this isn’t always possible. So you may want to “equalize” the coloring of the sapwood and heartwood.
One method is to bleach the wood using two-part bleach (sodium hydroxide and hydrogen peroxide). This will remove the coloring from the heartwood, so you can then stain the wood back to the color you want.
But it’s usually better to equalize the sapwood to the color of the heartwood.
The easier way to do this, if you intend to stain the wood dark, is to use a dark dye stain as shownin theaccompanying picture. Light colored dye stains don’t work as well,manned stains that contain binders, such as oil, water-based and lacquer stains, work even less well.
Though more difficult, the best way to equalize the coloring is to spray a dye stain just on the sapwood, as shown on the middle stripe in the second accompanying picture. You can then follow with a binder stain if you want to further equalize the coloring, as shown on the stripe second from the right. Finally, apply the finish, which will produce the true coloring.
Whenever water or any stain or finish that contains water comes in contact with wood, it causes the wood fibers to swell, which is called “grain raising” or “raised grain.” After the water has dried the wood feels rough to the touch, and thinly applied finishes also feel rough.
Raised grain occurs no matter how fine you sand the wood before wetting it. Because you can’t prevent raised grain if you use a water-based product, you need to deal with it so the final finish comes out smooth. There are two methods:
The first is to raise the grain and sand it smooth before applying the water-based product.
This is called raising the grain, sponging, whiskering or dewhiskering. Once sanded smooth, the grain won’t raise again nearly as much as it did with the first wetting.
After sanding the wood to about 150- or 180-grit, wet it with a sponge or cloth just short of puddling. Let the wood dry. Overnight is best, but three or four hours is usually sufficient if the air is warm and dry. Then sand the raised grain smooth with the same grit sandpaper you used last or one-numbered grit finer.
The goal is to smooth the raised grain without sanding deeper than necessary, in which case the newly exposed grain may raise again when wetted. Dull sandpaper works best because it doesn’t cut deeply so easily.
Raised grain is difficult to show in a picture, but it generally appears duller, as on the right side of the accompanying photo, which was wetted, dried overnight and not sanded smooth.
The second method is to “bury” the raised grain. If you’re applying a water-based finish, simply sand the first coat smooth after it dries, just as you would do with a solvent-based finish, except with a coarser sandpaper grit—for example, 220 grit. If you’re applying a water-based stain under a water-based finish, wait until after the first coat of finish has been applied and has dried to sand smooth. Sand lightly so you don’t sand through, which might remove some of the color.
When a finish changes from a liquid to a solid film, it’s called “drying” or “curing.” Although these terms are often used interchangeably, they refer to different methods of forming the film, and understanding this difference helps in understanding finishes.
Drying refers to the evaporation of the solvent, which results in a solid film. Shellac and lacquer are the most common finishes that change to a solid by drying. (Liquid and paste waxes also work this way.) Finishes that dry entirely by solvent evaporation can be redissolved by wetting the surface of the finish film with the thinner—alcohol for shellac or lacquer thinner for lacquer.
Curing refers to a chemical reaction that occurs in the finish to bring about the change from liquid to solid. Varnish (including polyurethane varnish) and all the two-part catalyzed finishes, including even two-part water-based finishes, cure this way. Though there might be an initial evaporation of the thinning liquid, once the chemical reaction has taken place, the finish can’t be redissolved with that thinner—mineral spirits, lacquer thinner or water.
Common one-part water-based finishes change from a liquid to a solid using both drying and curing. The individual particles in the finish cure, but they stick together to form a film when all the liquid (water and a co-solvent, which is usually a glycol ether) evaporates. Rewetting the film with water won’t dissolve it, but rewetting with the co-solvent will separate the particles causing a partial redissolving.
Pine is often stained to make it resemble a higher quality wood such as walnut, cherry or mahogany. You need to be aware, however, that the staining reverses the grain color, making what was the lighter-colored grain now the darker-colored grain. This happens because the softer, lighter-colored grain absorbs more of the stain than the much harder and denser darker-colored grain.
An example of this is shown in the first accompanying picture. The left side has been finished with a clear finish. The right side has been stained and finished.
The only way to keep this from happening is to spray the stain and not wipe off the excess. The problem is that this is similar to painting and obscures the figure of the pine.
An example is shown in the second picture where the same stain was applied and wiped off the left side and sprayed and not wiped off the right side.
Spraying and not wiping off solves the problem of grain reversal (and also blotching), but it produces an entirely different look.
If you transport unfinished wood or an unfinished wooden object through rain or sprinkling, the water drops will raise the grain, and wood stains will show up darker over these spots. The cause is the rough raised grain retaining more of the stain. The accompanying picture shows an example of what can happen.
There are two ways to avoid the spotty staining, both involving sanding (even if the wood has already been sanded). The first is to sand the wood thoroughly to remove the raised grain over the spots. The second is to wipe over all the wood with a wet cloth to wet it everywhere, then sand it smooth.
Because it’s hard to know how much sanding is necessary to cut deep enough to remove all the raised grain over the spots, the second method is actually the safer one because it creates an equal amount of raised grain everywhere. So even if you don’t sand enough to cut through all the water-damaged wood, the staining will still be even, maybe just a little darker.
It’s common to hear the instruction that it’s better to apply several thin coats than one thick one. Why is this so? Or is it?
What’s involved is drying time, nothing more. Thinner coats of all finishes dry faster than thicker coats. The difference is great enough that you can build the same thickness with several thin coats in less time than you can get that thickness with a thick coat. But the thick coat will eventually dry just as hard and perform just as well as many thin coats.
Shellac and lacquer dry entirely by solvent evaporation. The solvent evaporates much quicker from a thinly applied coat than from a thick coat because it takes much more time for the solvent at the bottom of the thick coat to work its way up through the finish and out into the air.
Varnish, including polyurethane varnish, dries by the crosslinking of the molecules in the presence of oxygen. It takes much longer for oxygen to work its way to the bottom of a thickly applied varnish than a thin varnish coat.
Water-based finish dries by both solvent evaporation and some crosslinking, so the result is the same. It takes much more time for a thick coat to dry than several thin coats.
Of course, oil finishes don’t dry hard at all, so coats must always be as thin as possible, accomplished by wiping off all the excess.