I hear it both ways. But I'm gonna agree with DH. A good quality adhesion primer will do the trick, but you do have to get to sound wood. I'm am going to differ on the brand (No offence meant to DH) and recomend using Brunning's "Grabber Plus". I've yet to find a latex product that works better.
Powerwashers are a great way to get rid of the gray wood, but be carefull with these machines, they meant it when they named them POWERwashers.