When the surface is cracked like alligator skin, it usually means one of three things: the paint is so old that it no longer has any resilience to changes in the weather, a newer top coat didn't adhere properly because it was not compatible with the primer or bottom coat, or, the paint was applied before a previous coat was dry. Avoid the latter by always using compatible paints and following label directions.

A badly cracked paint surface should be repainted, but before you do this, you will often need to remove the old paint.