What is osmosis in a GRP (fibreglass) hull, and what are the classic early signs to look for?

A. A cosmetic issue affecting only the deck, never below the waterline

B. Water gradually permeating through the gelcoat into the underlying laminate, reacting with residual chemicals to form a mildly acidic solution that draws in more water and can create small blisters — small, weeping blisters in the gelcoat below the waterline, sometimes with a vinegar-like smell when pierced, are the classic early sign

C. A structural defect that only occurs in wooden-hulled vessels

D. A problem that resolves itself permanently once a hull is lifted out of the water for a single season

A. Antifouling is a one-time application that lasts the lifetime of the hull

B. Antifouling works by slowly releasing biocides (or, for some types, gradually eroding) to deter marine growth — its effectiveness diminishes over a season or more as the active ingredient depletes, so it needs reapplying periodically, with the exact interval depending on the paint type, local water conditions, and how much the boat is used versus left static

C. Antifouling only needs reapplying if the hull colour visibly changes

D. Modern antifouling never needs reapplication once cured

A. There is no meaningful difference — both are always the same underlying problem

B. Osmotic blisters typically contain fluid (often with a distinctive smell) from the ongoing chemical process inside the laminate, while delamination is a separation between layers without necessarily involving that fluid or chemical process — correctly distinguishing them matters because the appropriate repair approach and urgency differ

C. Delamination only ever occurs above the waterline, so the two can never be confused

D. Osmosis and delamination are both purely cosmetic issues with no effect on hull integrity in any case