Definitely necessary if you leave it in any length of time. You will have a mess wit the algae. Some guys do their own and roll it on with the thicker green paint. With that method I think it is an every year deal. I have my dealer paint mine with the slick style paint. Get 3 or 4 years out of that. If I remember it is 450 or 500 bucks to have it done, so 150 - 200 year for my 22 footer.