Clean up the whole hull with either rubbing compound or polishing compound, depending on how oxidized it is, and then go over it with Poly-glo. $60 will do a thirty foot boat and will last at least a full year. It'll take you about 30-45 minutes to apply the Poly-glo.
The rest of your hull is probably chalky as well, due to breakdown of the gel-coat. The dark boot stripe, which is the contrasting band of color at the boat's waterline, shows the oxidation more because it's a darker color. I had to use #800 wet/dry paper on mine to clean it up as the rubbing compound wouldn't touch it. Since then, I spend about an hour and a half a year keeping my hull looking nice and shiny.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.