26sonic

I have an 02 31 sonic that is in fresh water . I keep it in for 2weeks at a shot and pull and clean it . there are 3 colors on this boat yellow, black + blue. and all go under the water line . why is my yellow paint starting to pimple ?this also happened on my 1998 26 sonic on the yellow only. Called lenny at sonic and they won't have anything to do with this problem because the boat was built in 02 and he did'nt own the company back then! any suggestions on what to do ? I love this boat and this is it's only problem.