time, or often no paint at all. When they do work they take hours, compared to minutes or seconds for dichloromethane-based strippers. Paint strippers come in a liquid, or a gel ("thixotropic") form that clings even to vertical surfaces. The principle of paint strippers is penetration of the paint film by the molecules of the active ingredient, causing it to swell; this volume increase causes internal strains, which, together with the weakening of the layer's adhesion to the underlying surface, leads to separation of the layer of the paint from the substrate. Other components Various co-solvents are added to the primary active ingredient. These assist with penetration into the paint and its removal and differ according to the target paint. Ethanol is suitable for shellac, methyl ethyl ketone is used for cellulose nitrate, and phenol and cresols are employed in some industrial formulas. Benzyl alcohol is used as well. Activators increase the penetration rate; for dichloromethane water is suitable, other choices are amines, strong acids or strong alkalis. The activator's role is to disrupt the molecular and intermolecular bonds in the paint film and assist with weakening this. Its composition depends on the character of the paint to be removed. Mineral acids are used for epoxy resins to hydrolyze their ether bonds. Alkaline activators are usually based on sodium hydroxide. Some cosolvents double as activators. Amine activators, alkalines weaker than inorganic hydroxides, are favored when the substrate could be corroded by strong acids or bases. Surfactants assist with wetting the surface, increasing the area of where the solvent can penetrate the paint layer. Anionic surfactants (e.g., dodecyl benzenesulfonate or sodium xylene sulfonate) are used for acidic formulas, cationic or non-ionic are suitable for alkaline formulas. Paint strippers containing surfactants are excellent brush cleaners. Thickeners are used for thixotropic formulas to help the mixture form gel that adheres to vertical surfaces and to reduce the evaporation of the solvents, thus prolonging the time the solvent can penetrate the paint. Cellulose-based agents, e.g., hydroxypropyl cellulose, are commonly used for mixtures that are not extremely acidic or basic; under such conditions cellulose undergoes hydrolysis and loses effectiveness, so fumed silica is used for these instead. Another possibility is using waxes (usually paraffin wax or polyethylene or polypropylene derivatives), or polyacrylate gels. Corrosion inhibitors are added to the formula to protect the underlying substrate and the paint stripper storage vessel (usually a steel can)