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Aircraft Maintenance TroubleShoot > Aircraft Paint Stits Poly Fiber and Randolph Blisters under finish coats1. Blisters are caused by moisture, air, or solvent vapors trapped under the primer, in the primer, or under the finish coat. An increase in temperature can cause the development of blisters months after the coating application. Moisture, air, or solvent vapors are trapped in many ways, including. A. Surface drying too fast.B. Urethane finish sprayed over thick lacquer primer or other one part coatings which soak up solvents, then slowly release after urethane surface has started to cross-link. C. Excessive high air pressure used to spray heavy coats which blasts air bubbles into the wet film. This can occur particularly on inverted or flat horizontal surfaces which are not prone to sag or curtain with excessive coating buildup. D. Excessive accelerator used in urethane finish coats to speed up the cross-linking when spraying in cool weather, trapping solvent vapors. E. Insufficient drying time between all coatings when spraying in cool, damp weather traps solvent vapors within the coatings. F. Primer or finish coat applied in high humidity which trap atmospheric moisture carried into the wet film with coating mist particles. G. Metal surface or primer surface not thoroughly clean after wet sanding, leaving microscopic waterborne salt particles with moisture on the surface. H. Condensation on the metal or primer surface when next coating
applied.
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