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Rubbing a Finish: The Key to Quality

Jun 18, 2016 | Expert's Corner | 0 comments

Of all the steps in finishing the one that people seem most hesitant about is “rubbing.” Yet rubbing a finish is as simple as sanding wood, and rubbing is the only step in finishing that can raise the quality of your work from average to special.

No matter how careful you are, you can’t apply a perfect finish. A brush always leaves brush marks, and a spray gun usually leaves some orange peel. Worse, there’s always some dust in the air that can settle and stick to the finish. Rubbing removes these flaws, and, in addition, improves the tactile qualities of the finish surface.

To overcome any fear you might have of rubbing a finish, apply several coats of a film-building finish to some scrap pieces of veneered plywood or MDF and practice. Try to rub through the finish on purpose to get a feel for how much rubbing it takes. Rub the surface to a gloss, and then cut it back with sandpaper or steel wool and start over. Approach rubbing a finish as you would cutting your first dovetail joint. Practice on scrap wood before trying it on a major project.

The Finish.

Film hardness is the quality in a finish that produces the best results when rubbed. Of the common film-building finishes, nitrocellulose lacquer is the easiest to rub to an even sheen—with the stress on “even.” This is one of the primary reasons lacquer continues to be used by the furniture industry on its finest furniture.

Varnish (including polyurethane varnish) cures tough and scratch resistant, but it is the most difficult finish to rub to an even sheen—precisely because it is so scratch resistant. You will get the best results if you allow the finish to cure for several weeks before rubbing. But the rubbed scratches will most likely still be coarser in some areas than others.

Whichever finish you use, let it cure to the point where you can’t smell any remaining solvent when you press your nose against the finish. Finishes that are rubbed before complete curing show scratches more and can even change sheen as the curing continues.

Rubbing With Steel Wool.

The simplest rubbed finish is one that is done with #0000 steel wool, or equivalent abrasive pad. Nothing more is involved than rubbing the surface in long strokes with the grain of the wood. As long as the strokes are straight and you keep an even pressure, the scratch marks you leave with the steel wool will soften and enhance the appearance and feel of the finish.

It’s unlikely you will rub through to the wood if you have applied at least three coats of a film-building finish.

But you have to be careful not to cut through the finish on edges. To avoid this, begin rubbing flat surfaces with short 3-inch to 6-inch strokes right up to the edge. Then connect these strokes with long straight strokes running the entire length of the surface, avoiding coming right up to the edge.

You can use a lubricant (soapy water, mineral spirits, mineral oil, or wax) with your steel wool, or you can rub without a lubricant. With water-based finishes you shouldn’t use mineral spirits, which can soften the finish.

Using any of the lubricants will reduce the coarseness of the steel-wool scratching a little. But if you cut through an edge, the lubricant will color the wood keeping you from noticing the damage, and you will make it worse. It’s best, therefore, not to use a lubricant until you have gained some experience using dry steel wool.

Rubbing with steel wool improves the appearance and feel of a finish, but it doesn’t remove flaws such as brush marks, orange peel or dust nibs. It just rounds them over. On tabletops subjected to harsh reflected light these flaws will still be noticeable. To remove them you have to level the surface before you rub it.

Leveling with Sandpaper.

You level a finish the same way you level wood to remove machine marks and other minor flaws—by sanding through several grits of sandpaper from coarse to fine. The difference is that you begin with a much finer-grit sandpaper (usually #400 grit or #600 grit), and you don’t need to sand in the direction of the grain except with the finest-grit sandpaper you intend to use.

In fact, it’s advantageous to change directions with each sandpaper grit, even sanding in circles if you like. This way you can easily see when you have sanded enough to remove all the scratches from the previous grit, and you are ready to move on to the next higher grit.

If the surface is flat and you want to keep it so, back your sandpaper with a flat cork, felt or rubber block. Otherwise, you can use your hand.

Begin sanding with #600-grit wet/dry sandpaper. To reduce clogging, use one of the lubricants mentioned above.

Sand until there are no more shiny spots caused by pores and other hollows. It’s easiest to see these spots if you use a gloss finish for all your coats. You will control the sheen with the final grit abrasive you use. Clean off the sludge now and then to check if you’ve sanded enough.

If #600 grit doesn’t cut through the problems fast enough, drop back to #400-grit sandpaper, or to #320 grit if the #400 grit is too slow. Then work back up through the grits. You can stop at #600 grit if you intend to rub with steel wool or with pumice. But you will usually get better results if you sand to #1000 grit first.

Because you are removing much more finish when sanding than when simply rubbing with steel wool, the finish should be thicker to begin with or you will likely sand through. Since people thin and apply finishes differently and the porosity of woods differ significantly (for example, mahogany vs. maple), it’s not possible to say exactly how many coats you should apply, but 3-to-5 should be adequate for close-grained woods and 4-to-6 for open-pored woods.

Rubbing After Leveling

It’s very difficult to get an even sheen using sandpaper, no matter how fine a grit you sand to. You almost always have to finish off by rubbing with steel wool or a rubbing compound.

Widely available and inexpensive pumice and rottenstone come in powder form, and you mix them with water, mineral spirits, or mineral oil to make a thin compound. You can do the mixing in a plastic squeeze bottle you use to spread the compound, or right on the surface you are rubbing. Rub the compound using a felt block or cloth wrapped tightly into a ball like a French-polishing pad. Rub in the direction of the grain.

Pumice produces a satin sheen that is a little shinier than that produced by #0000 steel wool. Rottenstone produces a soft gloss.

Commercial rubbing compounds in paste or liquid form usually produce a fairly high gloss. But you may have to experiment a little, because manufacturers don’t follow any standard grading system that would let you know in advance the level of gloss you will get.

When rubbing to a high gloss it’s critical that you keep everything as clean as possible. Any large grit that gets between your rubbing pad and the surface may leave a scratch deep enough to cause you to have to drop back a grit or two to remove.