Painting a barnacled bottom???

[ScowardNJ]

The previous owner of my boat bottom painted it while on blocks. Apparently there was no effort to prep and paint the areas under the blocks at the back corners and center keel. The last 2 years I have tried to remove as much barnacle crap off the 3 spots and paint over resulting in new barnacle growth both years. Any recommendations on how to remove the barnacle growth with out damaging the gel coat, and what to do about prepping so the bottom paint will stick?
Thanks!