Here's a before and after of an exterior painting job I did recently in Ilkley. The paint has curled away from the previous coats, this is due to the lack of preparation and sanding prior to painting the top coat.

This is a common result of poor decorating where a lack of attention to the preparation has meant the paint has not got a good enough key for the paint to adhere, now it has become my job to start over. Using a machine sander I have managed to rub the previous coats off back to a firm state and then applying a suitable primer to satisfy the porosity of the bare wood. Then exterior wood fillers were used to make the paintwork as smooth as possible, a quick rub down before applying the undercoat. Another undercoat and then top coat finishes off the paint system.