Annually
I apply a coat of VC17 every spring, in the fall I have very little of anything on the bottom and the powerwash remove most of the VC17, hence there is no buildup. The water in Collingwood is very clean, when I was in Lake Ontario, there would be a significant amount of slime in the fall, still only 1 coat a season did the trick and VC17 doesn't take long to apply. Sounds like and Ad for the stuff. Good Luck.