Bench / Projects / Bentwood Chair: Getting Sixty Years of Black Lacquer Off Steam-Bent Beech
Bentwood Chair: Getting Sixty Years of Black Lacquer Off Steam-Bent Beech
A Thonet-style bentwood chair that had been dipped in black lacquer at least twice, the second time without cleaning the first. Took a caustic tank to get it off, which is the only job here where caustic was the right answer.
Before
After
| Field | Value |
|---|---|
| Piece | Bentwood side chair |
| Estimated date | 1928-1938 |
| Substrate | Solid timber |
| Old finish removal | Caustic bath (tank) |
| New finish system | Pre-cat lacquer |
| Coats | 3 |
| Dry between coats | 4 h |
| Bench hours | 8 h |
| Materials cost | CNY 220-300 |
Why the caustic tank was the right answer here
Nine times out of ten a caustic dip is the wrong way to strip furniture. It raises the grain, it dissolves old hide glue, it darkens some timbers, and on anything with veneer it can lift the glue line. It is a shop tool for production work, not a home method.
This chair was the tenth case. Bentwood is solid beech, steam-bent, with no veneer and no glue line in the exposed parts. And the finish was two coats of black lacquer, the second applied over a dirty first without any key, so the whole thing was one continuous sheet of plastic sitting slightly proud of the wood. Solvent stripper sat on top of it and did nothing; the lacquer had cross-linked for sixty years and was effectively inert.
Time was the deciding factor. Gel stripper on bent parts with this much surface detail would have been six to eight hours of scraping in the radius sections. The caustic dip took 25 minutes and a neutralising rinse.
What the dip did and did not do
It took off every trace of the black lacquer, including the parts inside the bends where no scraper would have reached. It also took off the old hide glue at the two joints that had been repaired before, which turned out to be useful — those joints needed re-gluing anyway and the dip made the disassembly trivial.
It did raise the grain, visibly, which is expected and which is why the grit sequence starts at 120 rather than 220. It also pulled some colour out of the beech, leaving it paler than it would have been under the lacquer. That was accepted rather than corrected: dying bentwood back to a uniform tone looks like a modern reproduction.
Bench note: A caustic dip opens the wood. Anything that goes on top is going into a more absorbent substrate than usual, so the first lacquer coat needs to be thinner than normal, not thicker.
Disassemble while everything is still wet
The chair came out of the tank, got a 15-minute running-water rinse and a vinegar neutralising wash, and was then taken apart while the joints were still soft. Nine joints on this chair: four legs, two back posts, the seat ring, the back ring, the crest. Rubber mallet, light taps, no force.
Dowel joints that are still wet come apart cleanly. The same joints left to dry and re-set need drilling out, which turns a 20-minute job into two hours.
Dry it slowly and straight
Bent beech that has been through a caustic bath is at its most mobile. Left in a warm room it will move and it will not move back. This one hung in a 16 °C bay at 55 % RH for nine days, with the back posts and the back ring clamped to a straight batten for the first four.
Check it every day with a straight edge. If a bend has relaxed, it relaxes in the first 48 hours, while the wood is still malleable enough to be pulled back with light clamp pressure. After that it is set.
Deal with the two failed joints properly
Two of the leg sockets had been repaired before with a dowel and modern glue. The dowel had snapped. The socket was drilled out to 10 mm, a new beech dowel was turned 0.1 mm oversize, and the socket was reamed to match so the fit was a light tap, not a hammer.
Both were glued with hide glue at 60 °C. On a chair that is expected to be sat on, hide glue is the right choice: it is strong enough, it fails gradually rather than suddenly, and it comes apart again with heat and moisture if the repair ever needs redoing.
Grit ladder, starting coarser than usual
120 grit on the raised grain, then 180, then 220. It stopped at 220. On a bent part, going finer polishes the radius and makes it read as a hard edge rather than a curve, which shows badly on a pale timber.
Bent sections were sanded with a shaped cork block rather than a flat one, so the abrasive followed the curve. Twenty minutes with a bit of cork saves half an hour of hand-sanding in the radius.
Three coats of pre-cat lacquer, sprayed thin
HVLP, 1.3 mm nozzle, three coats with four hours between them. First coat thinned 30 % with lacquer thinner and sprayed almost dry — the wood is open after the caustic bath and a wet first coat will soak in unevenly and show as a blotch under the second.
Second and third coats at full strength, two light passes each rather than one wet one. A wet coat of pre-cat on an open substrate sags at the bends, and a sag on a bentwood chair is visible from across the room.
Wax the joints, oil nothing
Nothing is oiled on this chair. A wax block rubbed into each joint after the finish had cured kept the assembly moving as it should — bentwood chairs are designed to flex slightly at the joints, and a rigid, varnished joint is a joint that will crack.
The seat ring was left with one extra coat of lacquer compared with the rest, because it is the part that gets the wear.
One chair, one shop, one caustic tank that most people do not have. Caustic soda is genuinely dangerous — this is a record, not a recommendation, and information only.
Abrasive ladder
Grit sequence, coarsest first — each step removes the scratches of the one before.