Why is moisture avoidance in the air line important during priming bare steel?

• A. To speed curing
• B. To improve gloss
• C. To prevent flash rusting or blistering
• D. To reduce noise

Answer: (C)

Moisture in the air line matters because water on bare steel can cause rapid rusting and coating defects as the primer is applied. Bare steel oxidizes quickly when exposed to moisture, so any water carried by the spray air can form “flash rust” on the surface even before the primer cures. If the primer is sprayed over a surface with moisture or rust, the film can trap that moisture or rust beneath it, leading to blistering, poor adhesion, and pinholes. Keeping the air dry helps the primer wet the metal evenly, form a strong bond, and cure without those defects. This isn’t about speeding curing, improving gloss, or reducing noise—moisture would not help those and actually creates problems with the coating.