Priming

The foundation of every paint is the primer.  There is a different primer that is best to use for every  different surface.  If this part is not done correctly, the chances are it may never be right. 
On new or bare exterior wood, a slow drying oil primer is best.  The long drying time is allowing the product to soak into the wood to an extent that it does not easily fail.  This can then be lightly sanded and re-primed or just top-coated with two coats of any type of paint.
Caulking and window glazing should never be applied to unprimed wood.
PVC is no different and requires its own primer so the top coat does not peel away during the next pressure wash.
When possible, all wood should be back primed or box primed on all six sides before installing.
When priming tough to maintain surfaces such as some iron, industrial primers are available for the longest lasting finish.

Priming