Turpentine is obtained by distillation of wood from certain  pine trees. It is a flammable, water-insoluble liquid solvent  used as a thinner and quick-drier for varnishes, enamels, and  other oil-based paints. Turpentine can be used to clean paint  equipment and paint brushes used with oil-based paints.  Thinners include a plethora of solvents used to reduce the  viscosity of any one of the numerous types of primers,  subcoats, and topcoats. The types of thinner used with the  various coatings is addressed in other sections of this chapter.  Sometimes referred to as white spirit, Stoddard solvent, or  petroleum spirits, mineral spirits is a petroleum distillate used  as a paint thinner and mild solvent. The reference to the name  Stoddard came from a dry cleaner who helped to develop it  in the 1920s as a less volatile dry cleaning solvent and as an  alternative to the more volatile petroleum solvents that were  being used for cleaning clothes. It is the most widely used  solvent in the paint industry, used in aerosols, paints, wood  preservatives, lacquers, and varnishes. It is also commonly  used to clean paint brushes and paint equipment. Mineral  spirits are used in industry for cleaning and degreasing  machine tools and parts because it is very effective in  removing oils and greases from metal. It has low odor, is  less flammable, and less toxic than turpentine.  Varnish is a transparent protective finish primarily used  for finishing wood. It is available in interior and exterior  grades. The exterior grade does not dry as hard as the  interior grade, allowing it to expand and contract with the  temperature changes of the material being finished. Varnish  is traditionally a combination of a drying oil, a resin, and a  thinner or solvent. It has little or no color, is transparent, and  has no added pigment. Varnish dries slower than most other  finishes. Resin varnishes dry and harden when the solvents  in them evaporate. Polyurethane and epoxy varnishes remain  liquid after the evaporation of the solvent but quickly begin to  cure through chemical reactions of the varnish components. Naphtha is one of a wide variety of volatile hydrocarbon  mixtures that is sometimes processed from coal tar but more  often derived from petroleum. Naphtha is used as a solvent  for various organic substances, such as fats and rubber, and  in the making of varnish. It is used as a cleaning fluid and  is incorporated into some laundry soaps. Naphtha has a  low flashpoint and is used as a fuel in portable stoves and  lanterns. It is sold under different names around the world and  is known as white gas, or Coleman fuel, in North America.  Linseed oil is the most commonly used carrier in oil paint. It  makes the paint more fluid, transparent, and glossy. It is used  to reduce semipaste oil colors, such as dull black stenciling  paint and insignia colors, to a brushing consistency. Linseed  oil is also used as a protective coating on the interior of metal  tubing. Linseed oil is derived from pressing the dried ripe  flax seeds of the flax plant to obtain the oil and then using a  process called solvent extraction. Oil obtained without the  solvent extraction process is marketed as flaxseed oil. The  term “boiled linseed oil” indicates that it was processed with  additives to shorten its drying time.  A note of caution is usually added to packaging of linseed  oil with the statement, “Risk of Fire from Spontaneous  Combustion Exists with this Product.” Linseed oil generates  heat as it dries. Oily materials and rags must be properly  disposed after use to eliminate the possible cause of  spontaneous ignition and fire. The importance of primers in finishing and protection is  generally misunderstood and underestimated because it is  invisible after the topcoat finish is applied. A primer is the  foundation of the finish. Its role is to bond to the surface, inhibit  corrosion of metal, and provide an anchor point for the finish  coats. It is important that the primer pigments be either anodic  to the metal surface or passivate the surface should moisture be  present. The binder must be compatible with the finish coats.  Primers on nonmetallic surfaces do not require sacrificial or  passivating pigments. Some of the various primer types are  discussed below.  Wash primers are water-thin coatings of phosphoric acid in  solutions of vinyl butyral resin, alcohol, and other ingredients.  They are very low in solids with almost no filling qualities.  Their functions are to passivate the surface, temporarily  provide corrosion resistance, and provide an adhesive base  for the next coating, such as a urethane or epoxy primer.  Wash primers do not require sanding and have high-corrosion  protection qualities. Some have a very small recoat time  frame that must be considered when painting larger aircraft.  The manufacturers’ instructions must be followed for  satisfactory results. Red oxide primer is an alkyd resin-based coating that  was developed for use over iron and steel located in mild  environmental conditions. It can be applied over rust that is  free of loose particles, oil, and grease. It has limited use in  the aviation industry.  Wash Primers Red Iron Oxide Linseed Oil Mineral Spirits Naphtha Primers Varnish Thinners Turpentine 8-3