Thursday, June 3, 2010
French Polishing on a Wood Lathe
As my friends and collectors know, I am passionately committed to environmentally friendly finishes - what the FDA calls "GRAS" ("Generally Regarded As Safe.") For me, that usually means using pure, food-grade walnut oil on all pieces intended for direct food contact. For other work, I generally favor my own beeswax and walnut oil polish, the Real Milk Paint Company's "Dark Tung Oil" or Tried 'n True's "Original" finish. All of these products produce surfaces ranging from mat to a warm satin sheen, largely depending on the wood. "Hard" hardwoods like ash, oak, beech, and hard maples, when properly dried, can be sanded and polished so that a touch of walnut oil produces a bright, almost semi-gloss, finish. While the softer hardwoods like walnut and birch take on a softer mat glow.
Some work, like the candlestick set shown above, however, cries-out for a high-gloss finish. Problem is: most glossy finishes are anything but eco-friendly. (Yes, I know: after 72 - or so - hours, just about any commercial finish is considered "safe," but until then exposing oneself to mineral spirits and other VOC's is not something I care to do.)
Another technique, familiar to woodturners - especially pen turners - is called "friction polishing." Friction polishing is a fairly simple technique that uses a shellac-based finish that is applied directly via a rag held against the spinning workpiece. As the finish is applied, friction-induced heat dries it almost instantly. Although comparatively easy, friction polishing isn't foolproof. But with a little practice, one can obtain a pretty decent gloss finish. The nice things about friction polishing are (a) it's quick and (b) shellac is a natural product that, when dissolved in grain alcohol (I use 190-proof from the liquor store), fits into the GRAS category. (Shellac is also used as a pill coating, btw.)
The basic technique, which was more-or-less settled by the 18th century, is accomplished by using a wadded cloth pad called a "rubber" to hand rub the micro-coats of shellac and other materials into the wood surface. This is done in continuous circular and figure-8 patterns. Any resulting streaks are then "spirited off" with a fine application of alcohol - also applied with the rubber. The process is repeated - in various forms - 12 to 15 times over a period of two weeks to a month until the final finish at last emerges. (Fair warning: french polishing is anything but easy - and it sure ain't fast. Ask a luthier to french polish your custom guitar and you just added $1,500 - $2,000 to the tab and another month to your wait time.)
As we will see, the classic process is indeed labor intensive. The basic process works like this: the workpiece is first sanded to at least 600 - 800 grit to remove all toolmarks and all but the finest micro-scratches. Such fine sanding is needed because the french polish will magnify even the smallest defect(!) Next, we use our rubber to build-up a base coat shellac over 6 - 8 "bodying sessions" where we apply 2 lb cut shellac using continuous, overlapping circular and figure-8 motions. We can do 2 - 3 of these "bodying sessions" a day allowing 3 - 4 hours for the previous coat to dry and harden. (We should note that shellac takes weeks - if not months - to fully "harden." But 3 - 4 hours is sufficient for our purposes at this stage of the game.) After 3 - 4 days, having built up a reasonable shellac base, we fill the wood grain by rubbing the piece down as before, except now we add a drop of oil (I prefer walnut because I believe it gives a slightly harder final finish) and a dab of 4F pumice. (You can actually hear the pumice working with the shellac and wood fibers to fill the grain. Kinda neat, really.) We then set the piece aside for 2 - 3 days to harden-up.
Now the real work begins. Using our rubber and an equal amount of 2 lb shellac, (for me) grain alcohol, and a drop of walnut oil, we do the actual french polishing. This consists of another 6 - 8 sessions (at 2 sessions a day separated by 4 - 6 hours.) After each session, we "spirit-off" any streaks with our rubber moistened with a few drops of our grain alcohol. By the 3rd or 4th day, our workpiece shows some minor unevenness, which we level off with 800-grit sandpaper soaked in walnut oil. After another "spiriting-off," we follow-up with another 3 - 4 french-polishing sessions - this time with a 1 lb cut of shellac. (Almost done.) By now, we set our workpiece aside for another 3 - 4 days to allow the finish to stabilize and harden before a final sanding at 1200 - 1800 grit (again with walnut oil lubricant.) One more spiriting-off and we set the piece aside for 1 - 2 WEEKS to really harden. As a last step, I give the piece a final polishing with my home-brewed walnut oil/beeswax/rottenstone polish. E voila'! (I know of one artisan who uses a high-grade automobile polish - but since it ain't eco-friendly, I don't. No worries - to each his own....)
Fortunately, we woodturners can make things a little easier by using the lathe, rather than hand-power, at most stages of the game. One well-respected turner also advocates using thin CA glue as a base/grain-filling coat instead of shellac. (I have no experience with that technique, but it sounds promising.) In practice, I've found using the lathe makes each session easier, at least through the "rough-leveling." But it still leaves an uneven finish that must be rough-leveled by hand, as described above. (One could try power buffing, but I'm not crazy about it on a shellac finish.) I also find I get best results by doing the last french polishing sessions, with 1 lb shellac, by hand - with the final hand beeswax/walnut oil/rottenstone rubdown.
Candidly, I only invest this amount of time for special pieces like those shown. It is a lot of work, but the payoff - both in results and personal satisfaction - is more than worth it!
I hope you agree.
Posted by Brad Sears at 5:41 PM