Rubbing Out Wood Finishes
Reprinted with permission from Jeff Jewitt of Homestead Finishing Products.
Copyright ®2000 by Jeff Jewitt. All rights reserved. Not reproducible in any form, written or electronic, without permission.
Note: Abrasive grits referenced in this article are P (FEPA) graded. If you are using a CAMI grade abrasive, use the chart here to convert.
Finishes rarely look or "feel" right if left right off the brush or spray gun. Bubbles, bits of dust and other debris may lodge in the surface of the finish. You can feel these with your hand as you pass it over the surface. Brush marks and patterns from spray application will leave an irregular surface which is noticeable, particularly on gloss finishes. Rubbing out a finish is the last step in finishing and the object is to remove imperfections, even out and smooth the surface, and establish a consistent sheen to the finish.
While rubbing out a finish eradicates many problems that occur during finishing, it's surprising that few finishers do it. No doubt there is a lot of fear generated by the concept of sanding and rubbing a clear finish that's only six-thousandths of an inch thick. But rubbing out is a lot easier if you understand the theory of what you're doing and you do it in stages. This article will show you how to rub out any clear finish using both traditional and modern techniques. You'll be able to control the degree of luster (flat, satin or gloss) by the materials and techniques that you use.
Which Finishes to Rub
Satin, semi-gloss, flat and dull finishes are also not suitable for rubbing, although they can be abraded and then rubbed to remove imperfections. These finishes produce the various sheens -- satin, flat, etc., by the addition of fine silica which causes diffusion (scattering) of light at the surface of the dried finish. Abrading and then rubbing these finishes destroys this effect and may not product adequate results.
Abrasive papers - The traditional paper used for removing imperfections and leveling the finish are the silicon carbide wet/dry papers. They should be used with a lubricant. Rubbing oil, mineral spirits or Naphtha are used by many shops but some prefer to use soapy water with a dash of dish-washing soap added. This reduces the surface tension of the water and allows it to "wet" the finish surface better. Soapy water generally is less messy than using oil and a little easier on the hands.
For dry sanding, non-loading aluminum oxide or silicon carbide papers are used. They will clog quickly with finishes like oil based varnish, solvent lacquer, some water-based products, lacquers and shellac. On other finishes like acrylic (water and solvent based) polyurethane (water and solvent) and catalyzed varnishes and lacquer they don't clog as quickly. The advantage of dry-sanding is that you have better control because you can see what you're doing. Wet-sanding with a lubricant creates a slurry which gives a false illusion of a thick finish. It's easy to sand through to sealer or color coats with wet-sanding.
Several newer papers are available which are worth trying. They use new, proprietary processes to ensure precise, uniform grit size. Traditional processes allow as much as 35% deviancy from rated grit size. Mirka Royal series and 3M Fre-Cut Gold are two products which utilize uniform grit size. This makes them costlier, but they cut much faster and more efficiently. Both of these papers are "P" graded which is a different grading that you may be used to. In instances where I used to use a 400 grit standard (CAMI) paper, I use 600 grit or higher with the new papers.
Steel-wool, Synthetic steel wool - Steel wool is strands of steel spun into a pad. It is graded in the aught system from 3 (Very Coarse) to 0000 (Finest). The only grade I use is 0000.
Synthetic steel wool is a non-woven synthetic fiber pad that is impregnated with abrasive particles - usually aluminum oxide. Synthetic steel wool is graded by generic description - Coarse to Super Fine. Both of these act like abrasive papers, but because they are cushioned abrasive, they do not shear off high spots like dust pimples or even out irregularities in the finish surface. They ride over high spots. They are generally used after wet-sanding to establish a consistent scratch pattern for flat and satin finishes and with a lubricant - either soapy water, thinned wax or a lubricant made specifically for steel wool, called wool-lube. The gray color pad is equivalent to 000 steel wool and the white pad is impregnated with talc so it is used for polishing.
Abrasive powders - Abrasive powders are used for polishing up to the desired gloss after sanding and steel-wooling. Traditional powders for polishing are pumice (powdered volcanic glass) and rottenstone (powdered decomposed limestone). These mineral powders are composed of sharp, angular fragments that scratch the finish. Pumice is sold in different grades from 1F (Coarse) to 4F (Fine). Rottenstone is finer than pumice and is sold in only one grade which is fine enough to polish a surface to gloss.
The abrasive powders above can be combined with other materials to produce liquid suspensions or pastes which can be used by hand or machine. You can find these products at automotive supply shops. Two manufacturers, 3M and Meguiars, have systems of rubbing compounds that work very well on furniture finishes. However, we've found the Menzerna line of polishes we sell to be superior. If you use these products stay with one manufacturer - since a final rubbing compound from one manufacturer may be different in abrasive grit than another.
Stages in Rubbing Out
• Filling the pores - Gloss finishes look best when the surface appears as a flat, uninterrupted surface. If you plan on a gloss finish, fill the pores of open pored woods like mahogany, walnut and oak with paste wood filler. You don't have to fill the pores, but you'll have problems with some rubbing compounds getting stuck in the pores and drying white. (In which case use dark colored polishes like 3M's "Ebony" series) A gloss, open-pored finish is problematic to buff and doesn't really look as elegant as a filled-pored finish.
• Watch the edges - As some finishes dry there are forces at work which make them thinner at the edges. Finishes shrink as they dry and surface tension makes the finish shrink away from the edges towards the center. Surface tension is a force present in liquids which cause them to assume a spherical shape (like a drop of water) when in contact with a solid surface. Think of a finish as one large droplet of liquid - it actually pulls itself up away from the edges, much the same as a drop of water beads up on a glass surface. Although some finishes pull away worse than others, they all do it to a certain degree. Professional finishers learn to compensate for this phenomenon by spraying extra finish around the edges as they finish. However, if you're applying finishes by hand, this isn't possible, so take it easy.
• The finish itself - Since rubbing out removes finish, be sure to start with a thick enough one. The type of finish you use also has an effect. Solvent release finishes like shellac, solvent lacquer and some water-based finishes fuse together to form a single thickness of finish film. Reactive finishes like varnish and some two-part finishes do not melt into each other. This can pose a problem if you rub too much - you can rub through the last finish into the previous one and this creates a witness line. Fortunately, most reactive finishes have a higher solids content which means you have more dried finish build.
The greatest danger from rubbing through the finish topcoats occurs when you are leveling imperfections. To prevent this from happening use the following procedures:
Evaporative finishes (lacquers, shellac, and water-base) - Apply three coats to lock in the last coloring material or paste-wood filler application. Let dry one day, then level with 400 grit wet/dry paper. (See below- Leveling) Repair any small depressions, gouges, gum deposits etc., with stick shellac and re-level. Apply three full, wet coats after leveling.
Reactive finishes (varnish) -- apply two coats to lock in the last coloring material or paste wood filler and let dry until it will sand. Do any repairs as above, then level sand with 400 or 600. Apply one full wet coat.
This procedure provides a level base for the last topcoat(s) and the end result is that there's less finish to remove to level it.
• Let the finish cure - Cured finishes buff up better and faster than finishes that aren't fully cured. The longer you wait the better. Shellac, solvent and water-based lacquers and two part finishes should cure at least a week. Oil-based varnishes and polyurethane should cure at least two weeks.
If the finish is gummy and loads up the paper in the initial leveling, it's not dry enough. Let it cure longer.
Rubbing Out By Hand Using Traditional Materials
Removing imperfections - Start with the lowest grit wet/dry paper you can to remove the imperfections efficiently. I usually start with 400 or 600 grit but will go to 320 if the surface has big hunks of debris. Wrap the paper around a backing block like cork and sand just enough to remove the tops of pimples so that they're level with the rest of the finish. Soapy water can be used on most finishes but can cause problems with shellac and some water-based finishes. Use stearated aluminum oxide and dry-sand the finishes with a backing block. On textured surfaces with bad dust pimples, use your hand as a backing block and go easy. At this point the surface should have alternating dull and shiny spots when viewed in backlighting.
Leveling - Once the high spots, drips and other defects are removed, switch to the leveling step. The goal here is to establish a consistent scratch pattern across the entire surface of the wood. Try to do this step with 600 grit if you can. It makes the rest of the procedure go quicker. If the surface has rough brush marks or orange peel, you'll probably have to start at 400 or even 320. Either way, take some wet/dry paper and tear it into quarters. Wrap a quartered sheet around a cork block and squirt some soapy water onto the surface. I use a plant mister. Do the edges first. Work the edges all the way around the perimeter of the top with the paper with short, choppy strokes. Then work the center, working in one direction only (typically with the grain) Brush aside the slurry and look at the top in backlighting to check your progress. What you're aiming for is a surface with no shiny spots. Check the surface of the paper often- if it starts to show clogging, shift the paper to expose fresh grit or change the paper. Don't try to economize by using clogged paper, you'll create problems. If you set the paper down for any reason, set it on a clean surface. If it picks up any debris - it will scratch the fresh finish.
Brush aside the slurry and wipe the piece with naphtha if you want to accelerate drying of the water. When viewed in backlight, the entire surface should show a dull scratch pattern. More than likely you'll see areas that are still shiny. Squirt some more water on the surface and work these areas until but don't overdo it - you'll create a visible hollow. Work the shiny area a little then feather it in to the rest of the surface. Work slowly and deliberately until the entire surface is dull. If there are several small areas that are shiny - like partially filled pores, don't try to sand them level. They won't be too visible when the entire surface is buffed. If there are shiny spots around the edges and close to the edge, take some 0000 steel wool and using it dry, rub the surface until it's dull like the rest of the surface.
When you're satisfied with leveling, switch to the next grit wet/dry paper and do the same as above. Pay attention to the edges first then switch to the center of the board. Continue on to 800 grit.
Now you have a choice. If you want a satin finish take a pad of 0000 steel-wool, unravel it and fold it back into quarters. Wrap it around a cork block or soft piece of wood or plywood with cork glued to the bottom. Squirt some soapy water on the board and then apply some steel-wool rubbing lubricant (Behlen Wool-Lube) on the pad or the board. Rubbing with the grain, make 3 or 4 complete passes over the surface, slightly overlapping each pass with the next. Then switch to a fresh area of the steel wool and repeat. Do this 2 more times, switching to a fresh area of the steel wool for a total of 12-16 passes. Brush aside the slurry and check to see that you're putting down a uniform scratch pattern. You may have to let the board dry to see if you've got it right. If you want a waxed feel to the surface, let the wool-lube slurry dry, and then buff it off, just like wax. The surface will have a silky feel to it and when viewed in backlight, should look like brushed metal.
If you want gloss, skip the above step and continue wet-sanding up to at least 1200 grit. If you want to avoid a lot of work with the polishing compounds, take it to 1500 or 2000.
Now take some 4F pumice and sprinkle some on the surface of the wood. Squirt some water or rubbing oil over the pumice. Wad up a clean dry cotton cloth and working in whatever direction you want, polish every square inch of the board. Apply a good deal of pressure and replenish the pumice and water as it gets dry. Let the slurry haze over, then wipe it all off with a damp rag. Switch to rottenstone and do the same until the finish is as glossy as you want.
Rubbing to satin by hand is not that hard, but rubbing to gloss by hand is a lot of work. You can save a lot of time by investing in a power buffer if you rub out large surfaces. Power buffers are only effective on gloss finishes. Satin finishes still have to be hand-rubbed.
Rubbing Out Using Modern Materials
Start the process exactly like above with 400 or 600 grit paper. You can use 320 if the surface is badly orange-peeled, but only if you're sure you have a thick enough finish. Work the edges first, then switch to the hatching sequence and work up to 600 grit. If you want satin, you still have to do it by hand, using steel wool and wool-lube (You can substitute thinned wax if you want).
If you're aiming for gloss, work to at least 1500 grit. Working to a higher grit will make the polishing go quicker. Move the furniture to an area where flying compound won't be a problem and put an apron on to cover your clothes. Take some Menzerna 2L (or equivalent compounding paste) and smear some on the surface or squirt some stripes down the center of the surface about 8 inches apart. If you have an old pad on the buffer, spin off the old compound by holding a stick against it while it's spinning. I like to use 3M buffing pads, but Meguiar's makes foam pads which work just as well. With the buffer off, smear the compound all over the surface of the finish. With the buffer held off the surface, turn it on and holding it a very slight angle (about 2-3 degrees) place it on the surface of the finish. Move the buffer across the surface of the finish slowly, trying not to stay in one area too long. I like to work this sequence just like the wet sanding; I do the edges first, and then work the buffer into towards the center. (You can sprinkle the Menzerna 2L compound periodically with soapy water to keep it working.)
There is a natural fear to using a buffer on a clear finish. Try not to be too jerky and work the buffer in smooth, confident strokes. When working edges, pay attention to the angle and rotation of the buffer, sharp edges of the top may catch the edge of the pad and cause kick-back of the buffer. On large objects, like dining room tables, you can work the top while it's attached to the aprons and legs. On small, light items like nightstands you may need to remove the top and secure it with clamps or a vacuum hold-down.
The scratches from sanding disappear as you buff and it's easy to see when you're done with the compound. Overhead lighting or backlighting will highlight errant scratches and you can work any missed areas with compound. Let the compound haze, then wipe it off with a soft cloth and examine again. It's important to remove all scratches with the initial compound. If you don't and you discover scratches with the next compound, you'll have to go back and work up to the compound you're using.
After using the 2L (or equivalent), I switch to a polishing compound and repeat. You should see a deep gloss appear at this point. Cover the entire surface, let the compound dry, and wipe it off with a soft cloth.
Most finishers apply a glaze or swirl remover at this point for the ultimate gloss. I use swirl remover by hand or with the buffer. If you notice swirl marks from the buffing pad, you can use a swirl-remover, but I find that a light buffing with a clean dry pad on slow speed works just as well.
Note -- If you don't have a power buffer, you can always apply and work the polishing compounds by hand with a soft cloth. Or, if you have a random-orbit palm sander, purchase a foam pad that's sold as an accessory to fit on your Velcro sander bottom.
Menzerna Polishes will not create a whitish, hazy look on water-based lacquers. With other polishes this is usually due to the presence of aromatics in the mineral spirits which softens the lacquer, making it hard to polish. If this happens with a polish you're using switch to the Menzerna. I've never had a problem with these products.
Rubbing Out Difficult Surfaces
Turned legs - Use 0000 steel wool and wax thinned with mineral spirits. Avoid hard rubbing - you'll cut through the finish on sharp details.
Carvings - Carvings should not have a lot of finished applied in the first place- it destroys detail. When rubbing use steel-wool and wax and go lightly. Use dark colored wax on dark woods.
If rub-throughs happen during wet-sanding, you'll have to correct them before continuing. Stain and finish like in the above procedure, then very lightly dry rub the area with 0000 steel-wool. This should blend in with the satin finish. If you're going for gloss, you may want to use several more lacquer application before buffing.
How Sheen is Created
Interestingly, if you look at a satin surface from a low angle (like a dining table top from a kneeling position) it will start to appear glossy. That's because you're seeing less diffused light.
Reprinted with permission from Jeff Jewitt of Homestead Finishing Products.
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