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!
It’s a simple question. What do you do when checking out someone elses finish, whether in a store or at a friends home or shop? You run your hand over the finish. You will disapprove if it feels rough. If it feels smooth, you think, Boy, I wish I could do this.
A Great Finish is a Smooth Finish
There are many other factors that can be considered, such as thorough wood preparation to remove machine marks and dents, and even coloring problems discussed in previous articles. Smoothness is the key to a great finish.
You achieve a smooth finish by rubbing it. This is the only way to achieve a smooth finish. It is impossible to achieve a perfect finish with a spray gun, brush, or rag.
There are two significantly different types of finish: penetrating and film-building. Penetrating finishes don’t harden so you have to wipe off any excess after each coat. Penetrating finishes include oil finishes. Oil finishes include boiled linseed oil, tung oil, and a mixture of varnish and one or both of these oils.
Film-building finishes are all finishes that can harden. 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; Ill discuss both.
Oil Finishes
Sanding between coats with very fine grit paper (#320 grit) can make an oil finish that is quite smooth. Be sure to allow each coat to fully cure, which means leaving overnight in a warm room. Watco Danish Oil or Deft Danish Oil recommend that coats be applied within one hour.
Sanding the oil coats while they are still wet can make an oil finish that is smooth. Use very fine grit paper. Then wipe off the excess and allow whats left to cure overnight. Here’s how it works.
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.
All excess must be wiped away. Be sure to hang your wet rags to dry, or drape them singly over the edge of a trash can, so they cant spontaneously combust.
Let the finish dry in a warm place overnight.
Apply a second coat with oil to the surface and then sand it in the direction of grain using #600-grit dry/wet sandpaper. Use three to four back-and forth strokes to sand all areas. Theres 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.
Let the oil run off and let dry the surface overnight.
Apply another coat of oil, and sand again. Allow to dry overnight. You can finish the job with one coat.
A caveat: If you have stained the wood, sanding an oil finish wet or dry between coats is dangerous. You might sand through some of the color, especially at edges. Sand lightly and carefully.
Film-building Finishes
There are many film-building finishes, including varnish, lacquer and shellac. Both varnish and waterbased finish have a version called polyurethane. This is the regular finish (alkyd or
acrylic) with some polyurethane resin added.
One part of the pre-catalyzed laquer is also available with a catalyzed finish. Each of these finishes, except varnish, hardens in just a few hours in a warm place so you can apply multiple coats in one day. Varnish requires overnight drying between coats.
The Sealer Coat
The sealer coat is the first coat that you apply to any of these finishes. It seals the wood and stops the pores from getting worse. The wood will feel rough from the finish. Therefore, you must always sand the sealer coating smooth. You can skip the sanding, but it’s easier to sand your sealer coat as it is thin.
Varnish (not including polyurethane varnish) and lacquer are more difficult to sand than other finishes because they tend to gum up the sandpaper. Manufacturers offer a special product called “sanding sealer” to be used as a primer under such finishes. A sanding sealer is varnish, lacquer or a soap-like oil that has a lubricant. 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, its better to avoid using sanding sealer because it weakens the overall protection of the finish. This soap makes the finish softer by weakening the moisture barrier.
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). It is faster to sand the thinner finish layer because it hardens quicker.
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 is a sealer that blocks these problems, but it is much easier to sand. Theres 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
Its 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). Because it is less likely to clog, a dry-lubricated or stearated sandpaper works best. This sandpaper contains the same soaplike ingredient that is used in sanding sealer. It can usually be found at auto-body supply shops.
Sand just enough so you can no longer feel the dust nibs. Theres no reason to sand out brush marks or orange peel (caused by spraying) at this point.
Rubbing the Finish
Once you’ve applied all of the coats, which is usually three to five coats plus the sealer coat, it’s time for the surface to feel smooth.
If the dust nibs arent bad, you can usually improve the feel significantly by simply rubbing lightly with a folded brown-paper bag. The bag will not damage or alter the finish as long as it has dried properly.
To create a more perfect and attractive surface, rub it with #0000 steel wool or gray Scotch-Brite. Use the grain direction. Use short strokes to rub the 3-4 inches closest to the ends. This will reduce the chance of rubbing over the edges or cutting through. Continue to rub along the length of the hair, stopping just short of the edges.
A soap-and-water, mineral-oil lubricant containing the steel wool or Scotch Brite can help you achieve even better results.
Oil causes the abrasive pad to scratch the least, but cutting will be slower and the gloss attained higher. To find the best one, you can use both on the same surface. Most professionals use soap and water.
Be careful using any lubricant because if you cut through you wont see the damage until the lubricant dries. Rubbing with an abrasive pad, with or without a lubricant, improves the feel and appearance, but it doesnt remove the flaws; it just rounds them over and disguises them with fine scratches. You must first level the surface and then rub it.
Leveling and Rubbing
To level a finish, you follow the exact same procedure as sanding wooden floors. However, there are two key differences. You use finer grits and you use a lubricant to prevent clogging. Here’s the process.
To smoothen the surface, use a flat sanding board to support your sandpaper. Use a grit sandpaper that cuts through the flaws efficiently without creating larger-than-necessary scratches that then have to be sanded out, usually a grit between #320 and #600 (P400 and P1200).
Use wet/dry sandpaper (black in color) and a lubricant of mineral oil, mineral spirits or a mixture of the two. The lubricant you use will make sandpaper less likely to clog. (I find that sandpaper clogs quickly with a water or soap-and-water lubricant, but you can use one of these also.)
On unfilled, open-pored woods you may need to apply more than three coats so you dont sand through. Finishes have different solids and build and so everyone applies them differently. You will need to test your wood scraps to determine the right amount of coats to apply to avoid sanding through. Think in terms of four to seven.
A finish has no grain so you dont need to sand with the grain. In fact, you can sand in circles, which I find easier, and you can sand cross-grain near the ends (to keep from sanding through the finish at the edges).
Each time you advance to a finer grit sandpaper, change directions (circles, with the grain, across the grain) until you reach your finest grit which should go with the grain. You can remove the sanding fluid with naphtha.
mineral spirits, you will be able to see clearly when you have removed all the scratches from each previous grit sandpaper (a big advantage over sanding wood).
Gloss finish is better than satin. After sanding, use a plastic spreader to scrape away any sludge. If the surface has shiny spots or troughs, it is not level. When the surface is an even satin sheen overall, it is level and you can move to a higher grit sandpaper to remove the coarser scratches.
Once the surface is level, sand or rub it with finer and finer grit abrasives until you achieve the sheen you want. Begin by sanding up to at least #1000 grit, continuing to back your sandpaper with a fl at block or a felt or sponge pad. Use #0000 steel wool or pumice, and then rub the area with a mineral oil lubricant.
If you want a higher gloss, sand up to #2000 grit (P2000 or higher) and then rub with rottenstone and a mineral-oil lubricant using a felt or cloth pad. You can also use any other abrasive or rubbing compound.
One final word: I find that woodworkers often fear sanding the finish of a new project because they are afraid of damaging it. This is sort of like the fear of sanding veneer the first time. You have to do it to learn that it takes a lot of sanding to actually sand through. So with rubbing a finish, I suggest you first practice on a scrap piece of veneered plywood to gain confidence. To get an idea of how much sanding is required, apply several coats of finish to the plywood. PW
To view the PDF version of this article, click on the title.
Use rub to create a great finish
Bob is the author of Understanding Wood Finishing and a contributing editor to .
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