I have a csy to, never got a blister , what we do years ago is to remove the previous antifouling to bare gelcoat
and barrier coat the bottom with at least 12 coats of interprotect 2000e after moisture meter survey
and soundings test, interprotect come in 2 colors , white and grey, we use the grey color coat by coat, with the white we never put a full coat with the same amount as grey, no idea why...
My sugestion its if you reach bare gelcoat
or old primer stuff and found no blisters , for peace of mind barrier coat, and if you found blisters is something to consider, do it properly, peel the bottom or just fix the blisters..