The BOTTOM of my 79 o'day 22 has many small blisters from pee size to quarter size and some have a crack across them. Some places the blister cone has broken open, leaving a shallow hole. What preparation do I need to do with these blisters? Is this a gel coat surface that is blistering or part of the hull fiberglass? If I sand them down even with the rest of the hull it seems it would leave a thin surface with an air bubble underneath? When that problem is corrected what do I coat the surface with? COME ON David S