To create the smoothest oil finish possible, sand the second and each subsequent coat while it is still wet with finish (before wiping off) using #600-grit (P1200-grit) sandpaper, as shown above.
After spending countless hours building a project, you naturally want the finish to be perfect. To achieve that you need to understand the one thing that separates an OK finish from a great one. A great finish feels smooth!
Think about it. What do you do when checking out someone else’s finish, whether in a store or at a friend’s home or shop? You run your hand over the finish. If it feels rough, you disapprove (though you might not say
anything). If it feels smooth, you think, “Boy, I wish I could do this.”
A Great Finish is a Smooth Finish
Of course, there are also other factors, including thorough wood preparation to remove machine marks, dents and tear-outs, and achieving an even coloring – problems addressed in previous articles in this magazine. But when it comes down to it, the one factor that separates a great finish from an average finish is smoothness.
You achieve a smooth finish by rubbing it. This is the only way. You can’t get a perfectly smooth finish straight off a rag, brush or spray gun.
There are two significantly different types of finish: penetrating and film-building. A penetrating finish is one that doesn’t harden, so all the excess has to be wiped off after each coat. Oil finishes are penetrating finishes. Oil finishes include boiled linseed oil, tung oil, and a mixture of varnish and one or both of these oils.
All finishes that harden are film-building finishes. They can be built to a greater thickness on the wood by leaving each coat wet on the surface to dry. The procedure for rubbing is different for oil and film-building finishes; I’ll discuss both.
You can create a fairly smooth oil finish by sanding between coats using very fine grit sandpaper (#320 grit or finer). Be sure to allow each coat to fully cure, which means leaving overnight in a warm room. Some oil finishes, such as Watco Danish Oil and Deft Danish Oil, instruct to apply coats within an hour or two; following these directions won’t produce good results.
You can create an ultimately smooth oil finish by sanding each coat while it’s still wet on the surface using very fine grit sandpaper. Then wipe off the excess and allow what’s left to cure overnight. Here’s the procedure.
• Sand the wood to remove machine marks and other flaws.
• Wipe or brush on a wet coat of oil and keep the surface wet for several minutes, rewetting any areas that become dull because the finish has soaked in.
• Wipe off all the excess. Be sure to hang your wet rags to dry, or drape them singly over the edge of a trash can, so they can’t spontaneously combust.
• Allow the finish to dry overnight in a warm room.
• Wipe or brush on a second coat of oil and sand the surface while it’s still wet in the direction of the grain using #600-grit wet/dry sandpaper. Sand over all areas with three or four back-and-forth strokes. There’s no gain sanding more than this.
European standard “P-grade” sandpaper is rapidly replacing the American standard. Above #220 grit, P-grade numbers move up much faster than non-P-grade. Sandpaper of #600 grit is approximately equivalent to P1200 grit; #400 grit is about P800.
• Wipe off the excess oil and allow the surface to dry overnight.
• Apply a third coat of oil and again sand wet. Remove the excess and allow overnight drying. This is usually all you need to do to achieve an ultimately smooth finish, but you can repeat the procedure with a fourth coat, and with as many additional coats as you want.
One caveat: Sanding an oil finish wet (or even sanding dry between coats) is risky if you have stained the wood. You might sand through some of the color, especially at edges. Sand lightly and carefully.
Film-building finishes include varnish, lacquer, shellac, water-based finish and two-part catalyzed finishes. Both varnish and waterbased finish have a version called “polyurethane.” This is the regular finish (alkyd or
acrylic) with some polyurethane resin added.
Catalyzed finish is also available in one part called “pre-catalyzed lacquer.” Except for varnish, each of these finishes hardens within a couple of hours in a warm room so several coats can be applied in a day. Varnish, on the other hand, requires overnight drying between coats.
The Sealer Coat
The first coat you apply of any of these finishes is called the “sealer” coat. It stops up the pores and seals the wood. It also leaves the wood feeling rough, so you should always sand the sealer coat smooth. (Though you could skip the sanding and still achieve smoothness at the end by sanding just the last coat, it’s easier to sand the sealer coat because it’s thin.)
Varnish (not including polyurethane varnish) and lacquer are more difficult to sand than other finishes because they tend to gum up the sandpaper. So manufacturers provide a special product called “sanding sealer” to use as a first coat under these finishes. Sanding sealer is varnish or lacquer with a soap-like lubricant included. Sanding sealer powders when sanded.
If you are finishing a large project such as a set of cabinets with varnish or lacquer, it will be worthwhile to use a sanding sealer for your first coat. But if your project is small, requiring little sanding, it’s better to avoid using sanding sealer because it weakens the overall protection of the finish. The included soap weakens the moisture barrier and makes this layer softer than the finish itself.
Instead of using sanding sealer to gain easy sanding, you can thin the finish itself about half with the appropriate thinner (mineral spirits for varnish or lacquer thinner for lacquer). The thinner layer of finish hardens faster so it is easier to sand sooner.
If you are finishing a wood with resinous knots (such as pine), or you are refinishing wood with silicone contamination (it causes the finish to roll up in ridges) or animal-urine or smoke odors, use shellac as the sealer coat. Shellac blocks off these problems (but it is not easier to sand). There’s no reason to use shellac otherwise.
No matter what you use for the sealer coat, sand it after it dries using a grit sandpaper that creates smoothness efficiently without causing larger-than-necessary scratches – most often a grit between #220 and #400 (P220 and P800).
Sanding Between Coats
It’s always best to sand lightly between every coat of finish to remove dust nibs. This is done easily using very fine-grit sandpaper: #320 or #400 grit (P400 or P800). Using a “stearated” or dry-lubricated sandpaper is best because it clogs least. This sandpaper has the same soaplike ingredient as sanding sealer and is usually available at auto-body supply stores.
Sand just enough so you can no longer feel the dust nibs. There’s no reason to sand out brush marks or orange peel (caused by spraying) at this point.