Moisture is the reason why wood finishing turns white. Water gets locked inside the finishing when shellac or lacquer is sprayed in humid conditions. Polyurethane turns white after sanding - this milkiness is called blushing. Polyurethane doesn't usually blush, but it turns milky when it comes in contact with something wet over prolonged period of time. The procedure to correct the whiteness over a non-curing finishing is usually to wet the surface again with solvent so the water may escape. The procedure is more complex if the finishing is cured. Here are 5 ways to fix polyurethane that turns white after sanding: 1. Spray lacquer. You need to reduce the humidity of the room to 50% or even less with the help of dehumidifier spray. Mix one part lacquer and one part lacquer thinner and spray over the finish that is blushed. 2. Sand the finish. Use 220-grit sandpaper and sand the finish with light hands. Wipe the sand dust using a rag and spray a complete wet coat. Blushing should go away when the coating dries. If the whiteness still persists, add retarder to the mixture solution in the recommended proportion (as specified on the container) to reduce the drying time. 3. Use lemon oil or petroleum jelly. Spread a thick coating of petroleum jelly or lemon oil on the affected area of polyurethane finish and leave it for 24 hours. These substances have a high affinity and soak in to replace the locked moisture. Wipe the excess and allow the material to get dried. 4. Use a blush remover. You can use specialized product meant to deal with the blushing problem. For that, you can check out this video: 5. Warm the finish.
Use a lamp or blowdryer to warm the polyurethane so that it softens up and gets reconstituted. It should not be too hot to the touch. 6. Use denatured alcohol. Moisten a cloth with denatured alcohol and rub the white area gently. Alcohol has a tendency to absorb the moisture; however, you should use a moderate quantity of alcohol to avoid damaging the finish. Note: Wear a respirator and keep the room ventilated to avoid any negative health effects.
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