Whether you're stripping the finish off a chair you found at a garage sale or the layer of "antiquing" Mom put on that wonderful steamer trunk in the 1970s, you need to know what paint strippers are and how they work if you're going to be successful with as little effort and damage to the piece--and to your lungs--as possible. Choose one of three ways to remove paint, depending on the surface to be cleaned: mechanically, with heat or with chemicals. The object of all three methods is to separate the layer of paint or finish (such as varnish or lacquer) from the surface, or substrate, of the object we want to clean and refinish. Paint stripping is work no matter what technique is used and refinishing antiques generally reduces their value so it's wise to get a professional opinion on the value of an old piece before stripping it. Mechanical stripping--using tools with sharp edges to scrape the paint off and removing the top layer of wood with sandpaper--is the most labor-intensive technique. Using heat to remove paint literally "boils" the binder in the paint, softening the layer so that it can be removed by scraping. Neither technique is recommended for removing paint suspected to contain lead.
Chemical paint strippers use a wide variety of compounds to strip and clean surfaces. They are designed to "degrade" or break down the paint or finish or to destroy the adhesion of the coating to the substrate. They must accomplish this without damaging or chemically interacting with materials used in the surface. In addition, they must be free of chemicals that are highly flammable (high VOH) or poisonous and leave a surface that is easily re-coated. Methylene chloride is the active ingredient chosen for many types of chemical strippers, because its small molecules easily penetrate the paint layer and cause the coating to expand rapidly, pulling it away from the substrate. The bottom of the paint expands faster than its surface, causing separation that makes removal with stripping tools easy. Another type of chemical stripper uses alcohols or acetone, working as "co-solvents" to dissolve and dilute the finish. Co-solvents are used to remove "paint film," shellacs and some varnishes. Paint strippers designed for use on specific surfaces may also include chemicals known as "activators"--catalysts that increase the rate of penetration or dissolution. Surfactants wet the surface of the paint, thickeners allow strippers to cling to walls and colorants show you where the chemical is. Proper ventilation is important with any chemical stripper.
Even if you've used the right formulation for the paint and surface you want to clean, you'll need to go over the surface with a solvent, such as mineral spirits or turpentine, to clean off the "paint film," composed of pigments and solids that settle out as the softened or dissolved paint was removed. Some newer chemical strippers allow cleanup with water. Correct raised grain with a light sanding.