Strippers based on the solvent methylene chloride—also known as dichloromethane—have dominated the market for the past half century. They are speedy, nonflammable, effective on virtually all coatings except milk paint, and do not damage wood, metal, or masonry. Methylene chloride molecules are so small that they easily slip between the molecules in a coating's hard outer film, rapidly softening the film and releasing its grip. According to Thomas Wollbrinck, a paintings conservator who wrote about the composition of chemical strippers for the Journal of the American Institute for Conservation, the solvent works by swelling the paint to 10 times its original volume. The bubbles and blisters created from this swelling ultimately release the paint from the substrate.

The problem with these hyperkinetic molecules is that they also rapidly evaporate, which slows down the blistering process. To keep the solvent working on a finish as long as possible, manufacturers typically add paraffin, which forms a skin when exposed to air. That's why these strippers should be brushed on thickly—stroking only in one direction—over one small section at a time and then left undisturbed for about 15 or 20 minutes. At that point, if the paint is not removed or more stripper added, the finish will reharden and the paraffin will then inhibit additional stripper from working. After the old paint is scraped off but before new paint is applied, a wipe-down with denatured alcohol is the best way to remove any waxy residue. (Some manufacturers add surfactants—soaps—so that stripper residue can be washed off with water or mineral spirits.)

Methylene chloride's speedy evaporation rate also increases the risk of inhalation. Once breathed in, it metabolizes into carbon monoxide, which reduces the blood's ability to carry oxygen and makes the heart work harder. According to the EPA, short-term exposure can cause dizziness and lack of coordination; high exposures over an extended period can damage the liver and kidneys and may cause cancer. In an enclosed, poorly ventilated space, the vapor can overwhelm even the best organic–vapor respirators in as little as 15 minutes. Providing excellent ventilation is key to safety—then a respirator can do its job—but the only sure protection from vapors is to wear a respirator with an air supply. Methylene chloride that comes in contact with skin creates an immediate burning sensation (which can be neutralized with water), so wear long sleeves, trousers, and chemical-resistant plastic–laminated gloves. (Neoprene or butyl gloves can become ineffective in as little as six minutes.) Protective goggles are a must as well. Methylene chloride that gets in the eyes can cause severe burns and should be flushed out with plenty of water for several minutes. After that, the labels recommend, visit a doctor.
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