The splitting of 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 later stages, flaking occurs.
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 topcoat should prevent a recurrence of the problem.
Source: The Paint Quality Institute