for 1 hour. The total dry film thickness of primer should be 0.6 to 0.9 mil. If the temperature is below 70°F, you should allow 2 to 3 hours for drying. Do not spray if the temperature is below 50°F. Polyurethane Paint Systems All personnel assigned duties involving the mixing and application of polyurethane coatings should receive a preplacement and periodic medical evaluation. The date and results of each medical evaluation should be entered on the Administrative Remarks page of the individual’s service record and in the individual’s training jacket. The polyurethane systems used on naval aircraft consist of two types. The aliphatic type is used in MIL-C-83286 polyurethane paints. The aromatic type is used in MIL-C-85322 rain erosion-resistant coatings. These materials generally present no special hazard to health when they are cured (dried). They do require special precautions during their preparation, application, and curing because isocyanate vapors are produced. The untreated isocyanates released can produce significant irritation to the skin, eyes, and respiratory tract even in very small concentrations. They may also induce allergic sensitization. MIL-C-83286 aliphatic polyurethane is the standard general-purpose exterior protective coating for aircraft surfaces. Its unique combination of flexibility, gloss retention, and resistance to fuels and lubricating oils make the coating extremely suitable for aircraft exterior surfaces. It is supplied as a two-component kit of base and catalyst. You should use aliphatic polyurethane over epoxy-polyamide primer and for touchup and insignia marking over polyurethane paint systems. All personnel using polyurethane touchup operations should wear protective clothing as described in NA 01- 1A-509. Unprotected personnel should not be permitted closer than 15 feet to the spray zone during paint application with a brush, roller, or spray equipment. They should be permitted no closer than 40 feet during applications with compressed-air spray. Unprotected personnel should not be permitted closer than 15 feet to newly painted surfaces for 30 minutes after the painting operation is completed unless forced air exhaust ventilation is being used. Aliphatic polyurethane paint is available in kits consisting of 1 part pigmented material and 1 part clear resin component. When you mix aliphatic polyurethane paint, the clear resin component should always be added to the pigmented component. Only material from the same kit should be mixed together. However, two or more kits of the same color and manufacturer may be mixed in the same vessel. You should not mix clear resin components and pigmented components from different manufacturers. You should also follow the prescribed mixing ratios to prevent long drying times, poor chemical resistance, or loss of flexibility. You should use a mechanical shaker to agitate the pigmented component for at least 20 minutes. Then add the clear resin slowly to the pigmented component while you are stirring the pigmented component. Ensure the pigmented component and clear resin are thoroughly mixed. You should mix only the amount of paint that you can use in the 4-hour pot life of the mixed paint. When painting with polyurethane paints, you should clean the paint gun at the end of each use or every 4 hours, whichever comes first. To spray aliphatic polyurethane paint, you should thin it with MIL-T-81772 to the desired spray viscosity. Then stir the mixture, strain it through cheesecloth, and allow it to stand for a minimum of 15 minutes. If the viscosity of the mixed paint is too thick for spraying within 3 hours after mixing, it may be thinned again by adding MIL-T-81772 thinner. You should not attempt to rethin paint after 3 hours because it tends to produce orange peel or dry spots. Aliphatic polyurethane paint should be applied over a clean epoxy-polyamide primer within 8 hours of primer application. For the best results, you should apply the topcoat as soon as the primer is dry. You should apply the minimum thickness required to hide the primer. Apply two thin, wet coats about 30 minutes apart. Do not apply a mist coat because it may cause a low gloss. A primer or topcoat that has aged longer than 24 hours should be scuff sanded and cleaned before it is painted. You should allow approximately 8 hours for painted surfaces to dry. Additional time, usually 1 or 2 hours, will be required if the temperature is below 70°F. During the application of an aliphatic polyurethane topcoat, certain discrepancies may appear on the finish because of faulty application methods. The most common defects, probable causes, and preventions are listed in NA 01-1A-509. If any of these defects are found, they should be corrected before you continue to paint. 14-33