The above suggestions are useful, but wth a continued problem there is another angle to look at.
With zinc coating, your boat is in effect a huge anode. Any process that will cause an anode to be used up will also cause your zinc coating to be active and blow off the paint
I would check all your wiring
carefully to make sure that you do not have any stray electrical
currents around the boat.
Have you got enough anodes on the hull
It may be worthwhile to talk with an expert about using anodes that will protect the zinc. (Magnesium??)
I have found that buying
a silver chloride reference anode to check for the anodic activity of the hull
is useful. Not too expensive and should come with instructions for use.