Loss of paint due to poor adhesion. Where there is a primer and a top coat, or multiple coats of paint, peeling may involve some or all coats.

Possible Cause:

• Seepage of moisture through un-caulked joints, worn caulk or leaks in roof or walls.
• Excess moisture escaping through the exterior walls (more likely if paint is oil-based).
• Inadequate surface preparation.
• Use of lower quality paint.
• Applying an oil-based paint over a wet surface.
• Earlier blistering of paint (see blistering).

Solution:

Try to identify and eliminate cause of moisture. Prepare surface by removing all loose painting with scraper or wire brush, sand rough surfaces, prime bare wood. Repaint with a top quality acrylic latex exterior paint for best adhesion and water resistance.

Paint that has lost its adhesion to a galvanized metal substrate.

Possible Cause:

• Improper surface preparation, such as inadequate rust removal.
• Failure to apply a primer before application of an oil-based or vinyl latex paint.
• Failure to sand baked-on enamel finishes or glossy surfaces before painting. 

Solution:

Any rust on the metal should be removed with a wire brush; then, an acrylic latex corrosion-resistant primer should be applied (one coat is usually sufficient). New or previously painted galvanized metal that is completely rust-free can be painted with a top quality acrylic latex paint without applying a primer; however, a metal primer should always be applied to unpainted galvanized metal before applying an oil-based or vinyl latex top coat.

Source: Paint Quality Institute