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AC 43.13-1B 9/8/98 prints, but are usually the result of improper dope application. Pinholes are usually found in a second layer of fabric such as finishing tapes and reinforcing patches or over underly ing, non-porous structures; such as leading edges, turtle decks, and plywood surfaces. Any non-porous structure under fabric will act as a backstop and will resist complete dope penetration into the fabric. Microscopic cavi ties between the backstop and fabric collect escaping solvent vapors during the drying pro cess and balloon up through the surface leav ing pinholes, or become pinholes when the top of the balloon is sanded. Moisture, in the fab ric or on the backstop surface, also interferes with complete dope penetration, resulting in pinholes. The remedy for pinholes, at any stage before the final finish coat, is to add blush retarder to low viscosity dope and care fully brush over the affected surfaces to pene trate into and fill the pinholes. Discontinue brushing after five or six strokes to avoid leaving brush marks. c. Orange Peel. A rough spray gun- applied finish, similar to the texture of an or ange peel, may be caused by one or more of the following conditions: (1) Viscosity of material being sprayed is too high. (2) Air temperature is too high. (3) Spraying in direct sunlight, onto a hot surface or in a drafty/windy condition, which causes a fast solvent evaporation. (4) Spray gun, tip, cap, and/or needle are not properly matched for the type material being sprayed. (5) Volume of air available from the compressor not sufficient for spray gun. (6) Wrong thinner used and drying too fast. (7) Spray gun not properly adjusted. The spray gun should be adjusted to a uniform spray pattern with the material atomized to de posit fine, wet particles that merge and form a smooth film. d. Blisters. One or more of the following conditions my cause blisters: (1) Freshly coated surface placed in hot sunlight or high temperature area to accelerate drying time, causing the vapor from rapidly evaporating solvents to be trapped. (2) Excessive high air pressure used to spray heavy coats which “blasts” air bubbles into the coating. (3) Water or oil in air supply. e. Runs, Sags, and Curtains. These de fects may be caused by one or more of the following conditions: (1) Viscosity of material being sprayed is too low. (2) Coats applied too heavily. (3) Insufficient drying time between coats. (4) Spray gun held too close to work surface. (5) Improperly adjusted spray gun. f. Spray Gun Laps and Streaks. These defects may be caused by one or more of the following conditions: Page 2-30 Par 2-23PDF Image | AFS-640
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