The splitting of a dry paint film through at least one coat as a result of aging, which ultimately will lead to complete failure of the paint. In its early stages, the problem appears as hairline cracks; in its later stages, flaking occurs.
Possible Causes:
Use of lower quality paint that has inadequate adhesion and flexibility.
Overthinning or overspreading the paint.
Inadequate surface preparation, or applying the paint to bare wood without first applying a primer.
Excessive hardening and embrittlement of alkyd paint as the paint job ages.
Solution
Remove loose and flaking paint with a scraper or wire brush, sanding the surface and feathering the edges. If the flaking occurs in multiple layers of paint, use of a filler may be necessary. Prime bare wood areas before repainting. Use of a top quality primer and top coat should prevent a recurrence of the problem.
(image courtesy of The Rohm and Haas Paint Quality Institute)