Rubbing Out A Finish
What makes the difference between a good finish and a great finish? Rubbing out the finish. From the second you start rubbing the finish, you start to improve the surface tremendously.
The main objectives are to remove any small dust nibs that have gotten trapped in the finish before it set up, smooth out any remaining brush marks or over-spray, remove any other minor imperfections in the finish and finally achieve the desired level of sheen (from satin to high gloss). Some very good examples of rubbed finishes are: pianos that have a high gloss (mirror-like) look, electric guitars, and expensive conference tables. Rubbing does take time, but the extra time and effort are surely worth the results.
Many topcoat finish can be rubbed out successfully. Some will produce much better than others. There are two major qualities that determine if a finish can be rubbed well; the hardness of the finish after it has cured and whether multiple coats of a finish fuse together forming one layer.
Multiple coats of lacquer and shellac will fuse together forming one layer. These finishes cure brittle and hard making them easy to rub. The more brittle a finish after it cures (no matter how hard) the easier it will scratch and therefore the easier it will be able to rub using fine abrasives.
Conversion varnish and waterbased finishes also fuse together, although not as well as shellac and lacquer. These finishes cure tough and therefore do not scratch as easily as shellac and lacquer and are more difficult to rub out.
Finally, standard varnishes and polyurethanes also cure tough and therefore are harder to abrade than other finishes. What really makes these finishes difficult to rub out is that they do not fuse together. Each layer sits on top of the previous coat creating multiple layers. This is the reason you absolutely need to scuff sand between coats of these finishes, if you do not, each coat will not adhere to the previous one. When you rub a finish, you are cutting some of it away. If you cut through the top layer into a previous layer in some spots, you will leave a visible mark. These marks are called witness marks. These marks usually appear as irregular spots with a white ring around the perimeter. The only way to eliminate these marks is to completely rub through the top layer evenly exposing the layer below. This is very difficult do do, especially when the thickness of each coat is measured in thousands. If you have never rubbed out a finish, I suggest you start on one of the easier types to rub, such as lacquer or shellac.
Today, most finishes applied in cabinet and furniture shops are rubbed out using buffing machines. These machines or portable buffers have cloth or foam wheels that are charged with rubbing and polishing compounds. The process is much the same as when the paint job on the body of an auto polished, first with a coarser rubbing compound, then with polishing compound and finally waxed. This method speeds up the rubbing process. If you have a shop, or can afford a buffing machine, I suggest you look into this method. Because most of us do not have buffing machines, I will go over the manual rubbing process which was and still is used by many master craftspersons.
After the last coat of finish has been applied, you will need to set the workpiece aside for a time for the finish to cure before you can rub it out. Depending on the type of finish you have applied, the temperature, humidity and how many coats of finish you have applied, the finish can take anywhere from 36 hours to a couple of weeks to cure. I strongly suggest waiting as long as you can, especially if you are in a high humidity environment and you have applied more than 3 coats of finish.
1. Wet Sanding
The first step is to remove any dust nibs and smooth and level the surface. You will need 600 grit silicon carbide wet or dry sandpaper (black color) and some type of lubricant. Usually water or oil is used. You can purchase paraffin oil or rubbing oil from woodfinishing supply companies. Water will make the paper cut quicker, oil will slow down the cutting. I suggest you start with oil because it will be safer and there will be less of a chance of removing too much finish. If you cut through all the coats of finish in some spots, you have to start all over by sanding and applying more coats, so BE CAREFUL, especially on edges and corners where it can be very easy to cut through to the raw wood.
Apply a thin layer of oil to the surface of your finish. I usually pour a little on the palm of my hand and wipe it on the surface evenly. Next, take a 1/4 sheet of 600 grit wet or dry paper and fold it into three, keeping the abrasive sides outside. Now gently start sanding the surface taking long, straight strokes with the grain. Once you get to the trailing end of the surface, lift the paper right before the edge. The motion is that of a plane taking off a runway. This will prevent you from removing too much finish at the edge. After making one stroke, come back to the leading edge and start another pass, slightly overlapping the first. Continue this method until you have sanded the entire surface.
Periodically, you will need to wipe off the mix of oil and dust to check your progress. You will want to obtain a uniform sheen. Shiny spots are low areas that the paper has not touched yet. Apply more oil and continue to sand. You will need to sand more in order to level the finish enough to the point where the shiny spots are gone and the whole surface has a uniform sheen. Once this is accomplished, you should have a beautiful smooth, satin sheen. If you are happy with a satin sheen, stop here. All you will need to do is clean the surface with a rag slightly dampened with some mineral spirits and then apply a coat a paste wax or liquid polish if you wish. However, if you wish to obtain a higher sheen, you will need to continue the rubbing process using a finer abrasive such as pumice powder, which is covered in the next step.
2. Rubbing with Pumice Powder.
From here on, the only reason to continue to rub is to bring up a higher sheen or gloss. Wet sanding removed the dust nibs, leveled the surface and produced a satin finish. In order to produce a higher sheen or gloss, we need to use a finer abrasive. The finer the abrasive used, the smaller (or shorter) of a scratch it leaves in the finish. Although the scratches left by wet sanding with 600 grit paper are much too small to be seen with the naked eye, these scratches are still to large to produce a high sheen. Pumice powder is a very finely ground volcanic rock. Available in various grades. Usually the finer grades like FFF and FFFF are used in the rubbing process. I suggest FFFF pumice which is the finest of the grades. Pumice powder will will make the scratches left by the 600 grit paper smaller. The smaller the scratch, the more the light will reflect off the surface. The larger the scratch, the more the light gets trapped in the scratches producing a lower sheen.
For this operation you will need two felt blocks approx. 1/4 to 1/2 thick and 2 wide x 4 long, FFFF Pumice powder and more of the rubbing oil you used in the wet sanding operation. The felt block and pumice can also be purchased from woodworking or woodfinishing supply companies.
Apply some rubbing oil to the surface, and sprinkle a little pumice evenly over the oil. Next, using one of the felt blocks, start to rub gently with the grain using the same motion described for the wet sanding operation. Just like the wet sanding step, periodically wipe off the surface and inspect it for a uniform sheen. Re-apply the oil and sprinkle a little more pumice over the oil and continue. Once you have achieved a uniform semi- gloss sheen, you are done. Once again, wipe the surface clean and if you wish you can apply a coat of paste wax or liquid polish.
At this point, you have probably guessed that there needs to be one more step if you wish to obtain a high gloss finish. Yeah, you guessed right!
2. Rubbing with Rottenstone.
Rottenstone is also a very finely ground rock. It is even finer than pumicestone. Using rottenstone will produce the finest mirror-like finish. Rottenstone only is available in one grade and can also be purchased at woodworking and woodfinishing stores. The process is exactly the same as the pumice process, just make sure you do not use the same felt block that you used for rubbing the pumice, if you do, the pumice will mix with the rottenstone and give you an uneven scratch pattern. Once done, again, clean and apply paste wax or liquid polish.
The wet sanding, pumice, rottenstone is not the only method for rubbing out a finish. As I mentioned earlier, one may use rubbing and polishing compounds along with a buffing machine. There are also superfine abrasive papers that can be used in place of the above methods. One of these is called Micro Mesh which is a series of abrasive snadpapers that have a rubber and cloth backing. These papers start at 1500 grit and go up to 12,000 grit, leaving a scratch pattern that is so uniform and small that it produces a super mirror gloss finish. I plan to cover the use of this paper in a future article.
All finishing and rubbing materials in this article can be purchased from:
A Constantines & Son Inc.