Blotchy Stain and Milky French Polish on Twelve Mango Stools
A workshop must deliver 12 mango stools and is behind schedule. The stools were sanded only to 80 grit. A water-based walnut stain was wiped on and left to dry in a room at 22 °C and 85% relative humidity. The result is patchy: some areas are almost black, the grain lines are still pale, and the surface feels rough. The polisher then applied a French polish using shellac flakes from a tin opened in 2019 and methylated spirit from an unsealed bottle. Two days later the film has white, milky patches and still feels soft under the thumb. The polisher proposes to sand the milky patches off and put on more shellac, thicker this time, one coat every 20 minutes to save time.
1. Diagnose the blotching and the rough surface and set out the re-preparation needed before any more finish is applied.
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Cause of the blotching. Mango has hard, dense latewood bands and soft, open earlywood. Finishing at 80 grit leaves deep scratches and crushed fibres; the soft earlywood drinks stain while the dense bands take up very little, so the colour goes dark where the pores are open and pale on the hard bands - the grain lines staying pale is the clue. The water in the stain raises the crushed fibres, which is the roughness felt afterwards, and drying at 85% RH is so slow that the stain stays wet long enough to wick along the grain and dry unevenly.
Re-preparation.
- Remove the present stain as far as possible with a cabinet scraper and 120 grit. Stain that has already dried unevenly cannot be evened out by adding more.
- Sand through the grits with the grain: 80 → 120 → 180 → 220. Each grit must remove the scratches of the one before; skipping a grit leaves scratches that show through the finish.
- Raise the grain deliberately: wipe with a damp cloth, let it dry, and re-sand with the last grit used. That moves the raised fibres to before the finish instead of after it.
- Seal before staining: a thin shellac size (about 1 part shellac to 6–8 parts spirit) or a commercial sanding sealer, de-nibbed with 320 grit. This equalises absorption between earlywood and latewood.
- Stain thinly with a rag, working small areas, wiping excess off across the grain and then lightly along it. A gel or wiping stain is far more forgiving on mango than a water-based dye, and a dye thinned and built up in two coats gives more control than one strong coat.
2. Explain the milky patches and the soft film. List the causes in order of likelihood and say what must change in materials, room conditions and technique; also say why the polisher's proposal would make matters worse.
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What milky shellac is. Milky patches are almost always water trapped in the film; the film then scatters light instead of transmitting it. Softness is trapped solvent, or shellac that has chemically aged.
Causes, in order of likelihood.
- Room conditions. At 85% RH and 22 °C the evaporating spirit cools the surface below the dew point and water condenses into the film. French polish needs roughly 20–25 °C and below 65% RH.
- Spirit. An unsealed bottle of methylated spirit absorbs atmospheric water; once diluted it carries that water straight into the film and dries slowly.
- Shellac. Flakes opened in 2019 have absorbed moisture and begun to polymerise; old shellac dries dull, stays soft and loses adhesion. Flakes keep about 3–5 years if sealed and dry, and a mixed batch is good for weeks, not months.
- Technique. Coats every 20 minutes are far too close. Each coat must lose its solvent before the next or solvent is trapped and the film stays soft and cloudy.
Why the proposal makes it worse. Sanding back and then applying thicker coats faster simply buries a wet, contaminated film under more shellac and traps more solvent and water. It may look acceptable for a week and then go cloudy or never harden at all.
What must change. Strip the soft film with 320–400 grit and a little spirit and let the wood dry. Use fresh dewaxed shellac in fresh industrial methylated spirit at about a 1:4 to 1:6 cut; test a drop on glass - it should dry clear and hard within 30 minutes, otherwise discard it. Dehumidify the room or wait for a dry day, apply thin coats 2–4 hours apart, and keep the work dust-free.
3. Give a complete finishing schedule from bare wood to final wax for these stools, with grits, materials and approximate drying times.
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- Sand: 80 → 120 → 180 → 220 with the grain; scrape awkward corners rather than sanding across the grain.
- Raise the grain: wipe with water, dry about 4 hours, re-sand at 220.
- Sealer/size: one thin coat of 1:8 shellac or a sanding sealer; dry 2 hours; de-nib with 320.
- Stain: one thin coat of wiping stain, ragged off across and then along the grain; dry 8–12 hours. A second very thin coat only if the colour is too light - check against an approved sample, not against memory.
- Fill the pores: mango has open pores that will otherwise show as dimples in a polished film. Work pumice powder and shellac across the grain, or use a paste filler tinted to the stain; dry overnight; sand back with 400.
- Body coats: 3–5 very thin shellac coats applied with a rubber (cotton wool in a linen wrap), overlapping circles and figure-eights, each 2–4 hours apart, in a dust-free room below 65% RH.
- Flatten between coats: 600 grit used dry with a little wax, or 4/0 steel wool; dust off thoroughly.
- Spirit off: a barely damp rubber with a drop of spirit to melt the surface haze and take off the oil.
- Cure and wax: 24–48 hours, then a thin coat of furniture wax, buffed with a soft cloth.
Finish one complete offcut from the same batch first. Twelve stools is not the place to discover a bloom, and a test piece also tells the polisher how many body coats this particular timber needs.