It depends how bad the blisters are and how many..osmosis is a pain to fix properly.
To do it right u need to dry out the underwater body. I use infrared heaters wich I place around the hull
..the drying out process can take a few days depending on the moister contents in the lay up. use a moister meter to check. Next step is to remove the entire gel coat, grind out the blisters, fill with an epoxy
filler sand everything smooth. Apply 2 coats of clear epoxy
like West System and 2 coats of an epoxy 2 pack primer..Epinamel made by Wattyl is a good choice.
If the osmosis
is realy bad the underwater body has to be reglassed with glass/epoxy or Vinyl Ester.
If u just grind out the blisters and don't dry out the substrate it wont last.
molecules are in the lay up and were introduced when the hull
was laminated and the humididty was not controlled.
The gel coat acts as a membrane..hence osmoses