I remember there was a debate on this last year. But I'm not sure what
verdict we arrived at. My deck is about a year old and the PT web site
says that is the time to add waterproofing. Linseed oil, I believe was
the specific ingredient they mentioned to use.

I live in a development with alot of these types of decks and I'd say
about 20 percent of the people don't do anything with them.. If it's
going to get me 30 years out of a deck instead of 25, I'd just as soon
leave it alone. Is it going to add considerably to the life of the
deck..
TIA
CP