Pettit barnacle buster vs rustoleum cold galvanizing compound

I used Propspeed the first year I bought my new-to-me boat. I'm in Long Island Sound at a slip. When the boat was hauled at the end of the season, the prop and shaft were covered in barnacles. For the price ($300 for 200ml), I expected better; a lot better. Last year, I tried cold galvanizing zinc spray paint. I think the can was $8 at the local hardware store. Not knowing any better, I put on the equivalent of a coat and a half of that stuff. At the end of the season, I was disappointed to see barnacles; not as many as with the Propspeed, but still more than I had expected. This year, I put three full coats of the zinc paint, following the directions on the can for drying and reapplying. I'm hoping that gives improved results. My friend was also painting the shaft and prop on her boat at the same time. She recommended the Petit spray paint she uses and swears by. I looked it up when I got home. 93% zinc.