Cause
Unprotected wood which has been exposed to the elements (U.V. light) and has become extremely dry causing a breakdown of the natural cells.
Mechanically sand down to remove the top layer of wood followed by priming with an oily wood primer. Standard primers if used should be thinned to obtain penetration of the wood,
Standard alkyd systems can then be applied.
NB. If the de-natured wood is not removed and replaced then further paint failures are likely.