Urethane  Gray Enamel Undercoat Epoxy Dope Identification of Paints Lacquers $^{ }$Synthetic Enamel Zinc Chromate This is a single component, nonsanding primer compatible  with a wide variety of topcoats. It fills minor imperfections,  dries fast without shrinkage, and has high-corrosion  resistance. It is a good primer for composite substrates. This is a term that is misused or interchanged by painters  and manufacturers alike. It is typically a two-part product  that uses a chemical activator to cure by linking molecules  together to form a whole new compound. Polyurethane is  commonly used when referring to urethane, but not when  the product being referred to is acrylic urethane.  a colloidal solution of cellulose acetate or nitrate combined  with plasticizers to produce a smooth, flexible, homogeneous  film.  Dope is still used on fabric covered aircraft as part of a  covering process. However, the type of fabric being used  to cover the aircraft has changed. Grade A cotton or linen  was the standard covering used for years, and it still may  be used if it meets the requirements of the Federal Aviation  Administration (FAA), Technical Standard Order (TSO)  C-15d/AMS 3806c.  Polyester fabric coverings now dominate in the aviation  industry. These new fabrics have been specifically  developed for aircraft and are far superior to cotton and  linen. The protective coating and topcoat finishes used  with the Ceconite$^{®}$ polyester fabric covering materials are  part of a Supplemental Type Certificate (STC) and must  be used as specified when covering any aircraft with a  Standard Airworthiness Certificate. The Ceconite$^{®}$ covering  procedures use specific brand name, nontautening nitrate and  butyrate dope as part of the STC. Urethane primer, like the urethane paint, is also a two-part  product that uses a chemical activator to cure. It is easy  to sand and fills well. The proper film thickness must be  observed, because it can shrink when applied too heavily. It is  typically applied over a wash primer for best results. Special  precautions must be taken by persons spraying because  the activators contain isocyanates (discussed further in the  Protective Equipment section at the end of this chapter). Epoxy is a synthetic, thermosetting resin that produces  tough, hard, chemical-resistant coatings and adhesives. It  uses a catalyst to chemically activate the product, but it is not  classified as hazardous because it contains no isocyanates.  Epoxy can be used as a nonsanding primer/sealer over bare  metal and it is softer than urethane, so it has good chip  resistance. It is recommended for use on steel tube frame  aircraft prior to installing fabric covering.  The Poly-Fiber$^{®}$ system also uses a special polyester fabric  covering as part of its STC, but it does not use dope. All the  liquid products in the Poly-Fiber$^{®}$ system are made from  vinyl, not from cellulose dope. The vinyl coatings have  several real advantages over dope: they remain flexible, they  do not shrink, they do not support combustion, and they are  easily removed from the fabric with MEK, which simplifies  most repairs.  Zinc chromate is a corrosion-resistant pigment that can  be added to primers made of different resin types, such as  epoxy, polyurethane, and alkyd. Older type zinc chromate  is distinguishable by its bright yellow color when compared  to the light green color of some of the current brand primers.  Moisture in the air causes the zinc chromate to react with  the metal surface, and it forms a passive layer that prevents  corrosion. Zinc chromate primer was, at one time, the  standard primer for aircraft painting. Environmental concerns  and new formula primers have all but replaced it.  Synthetic enamel is an oil-based, single-stage paint (no clear  coat) that provides durability and protection. It can be mixed  with a hardener to increase the durability and shine while  decreasing the drying time. It is one of the more economical  types of finish. The origin of lacquer dates back thousands of years to a resin  obtained from trees indigenous to China. In the early 1920s,  nitrocellulose lacquer was developed from a process using  cotton and wood pulp.  When fabric-covered aircraft ruled the sky, dope was the  standard finish used to protect and color the fabric. The dope  imparted additional qualities of increased tensile strength,  airtightness, weather-proofing, ultraviolet (UV) protection,  and tautness to the fabric cover. Aircraft dope is essentially  Nitrocellulose lacquers produce a hard, semiflexible finish  that can be polished to a high sheen. The clear variety yellows  as it ages, and it can shrink over time to a point that the surface  crazes. It is easy to spot repair because each new coat of  lacquer softens and blends into the previous coat. This was  one of the first coatings used by the automotive industry in  mass production, because it reduced finishing times from  almost two weeks to two days.  8-4