Blotchy Teak-Finish Tables in a Humid Workshop
A school workshop has an order for 12 reading tables. Each top is 1200 x 600 x 25 mm; each table has four legs 50 x 50 x 750 mm and aprons 75 x 25 mm. The client wants a “warm teak look” with a low-sheen finish and delivery in four weeks. The workshop has sal and gamari in stock and a materials budget of ₹45,000. In the previous batch of five tables, a water-based stain was applied, followed by two coats of solvent-based lacquer. After two weeks, the finish showed white blush and blotchy pale patches, especially along glue lines and end grain. The workshop humidity is 85%, temperature 28–32°C, and there is no heated spray booth. The instructor proposes to use the same stain and lacquer but to add more thinner.
1. Explain the likely causes of the white blush and the blotchy patches. Why is adding thinner to the lacquer not a reliable cure?
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White blush in a lacquer film is usually moisture trapped in the film. In a humid workshop, the fast-evaporating solvent cools the wet film below the dew point, and water vapour condenses into the finish. The trapped water scatters light and looks white or milky. Adding thinner makes the film evaporate faster and cool more, so it can make blushing worse rather than better.
The blotches have a different cause. A water-based stain raises the grain and is absorbed unevenly, especially on end grain, which drinks stain and turns dark. Glue squeeze-out that was not sanded off seals the pores, so the stain cannot penetrate there and those areas stay pale. Inconsistent sanding has the same effect. The result is a patchy, mottled surface rather than a uniform teak colour. Adding thinner to the lacquer does nothing to fix glue contamination or uneven stain absorption; it only changes the drying behaviour of the topcoat. The correct approach is to control the surface preparation and the finishing environment, and to choose a finish system suited to the humidity.
2. Propose a finish schedule for the 12 tables that will give a warm teak look and a low sheen in this workshop. Include surface preparation, stain or sealer choice, and number of coats.
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Sand progressively to 80, 120 and 180 grit, then remove all dust. Sand off every trace of glue squeeze-out; a raking light helps. Seal the surface with a thin coat of dewaxed shellac or a commercial sanding sealer, especially on end grain, to equalise absorption. For the teak look, use a pigmented oil-based wiping stain or a gel stain rather than a water-based stain; gel stain sits on the surface and is less prone to blotching. Test on offcuts of both sal and gamari, because the two species will take the colour differently. Wipe off Excess stain and allow it to dry fully.
For the topcoat, choose a satin oil-based polyurethane or a pre-catalysed satin lacquer applied under controlled conditions. Two thin sealer coats followed by two or three thin topcoats are better than one thick coat. Add a retarder to the lacquer if spraying in high humidity, and provide cross-ventilation without blowing dust onto the wet film. Do not spray if the relative humidity is above about 75%; brush or wipe the oil-based finish instead. Rub the final coat lightly with 320-grit paper or fine synthetic pad to remove dust nibs, then apply a thinned final coat if a uniform low sheen is required. Keep the tables in a dry room for at least 48 hours before delivery.
3. Estimate the wood and finish quantities for the 12 tables. Show your method and state the allowances you would use.
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Calculate the net volume first. Tops: 1.2 x 0.6 x 0.025 = 0.018 m³ each, so 12 tops = 0.216 m³. Legs: 0.05 x 0.05 x 0.75 = 0.001875 m³ each; four legs per table = 0.0075 m³; for 12 tables = 0.09 m³. Aprons: assume two long rails 1.1 m and two short rails 0.5 m, all 0.075 x 0.025 m. Long rails: 2 x 1.1 x 0.075 x 0.025 = 0.004125 m³; short rails: 2 x 0.5 x 0.075 x 0.025 = 0.001875 m³; per table = 0.006 m³; for 12 = 0.072 m³. Net total = 0.216 + 0.09 + 0.072 = 0.378 m³. Add 20% for sawing, planing, defects and waste: 0.378 x 1.2 = 0.454 m³, which is about 16 cft or 192 board feet.
For finish, estimate the exposed area. Each top has about 0.72 m² on top, 0.72 m² underneath and 0.09 m² of edge, say 1.5 m². Legs and aprons add about 1.2 m² per table, so allow about 2.7 m² per table, or 33 m² for 12 tables. At a sealer coverage of 8 m² per litre per coat, two sealer coats need about 8.3 litres; at 10 m² per litre for two topcoats, allow about 6.6 litres. Add 10% for loss. At an assumed ₹1,800 per cft for sal, the wood cost is about ₹29,000; finish, adhesive and abrasives may take ₹8,000–10,000, leaving the ₹45,000 budget tight but feasible if the offcuts are used for the aprons.