Can I repair minor fibreglass osmosis blisters myself?
I hauled out my 1990s cruiser for the winter and found about a dozen small blisters on the hull, mostly around the waterline. They smell a bit like vinegar when popped. Can I fix these minor osmosis blisters myself without paying a yard thousands for a full peel?
Answer
Yes, you can repair minor localised osmosis blisters yourself using a small grinder and a marine epoxy system. The secret to a lasting repair is ensuring the exposed fibreglass is completely dry and salt-free before you apply filler or a barrier coat.
Grind and wash blisters
Put on a respirator, safety goggles and overalls to protect yourself from fibreglass dust. Use a small angle grinder with a 60-grit flap disc to open up each blister. A Dremel tool with a sanding drum works well for tiny spots. Grind past the gelcoat and into the laminate until you reach clean white fibreglass. Chamfer the edges of the crater to form a shallow dish shape. This gives the epoxy a larger surface area to bond to.
The fluid inside an osmotic blister contains glycol and acetic acid, giving off a distinct vinegar smell. This acid prevents epoxy from curing. Wash the ground-out craters thoroughly with fresh water and a stiff scrubbing brush. A pressure washer also removes chemical residue. Do not use solvents like acetone at this stage because they push contaminants deeper into the laminate. Wash the craters two or three times over a week.
Dry the hull laminate
Many DIY repairs fail here due to impatience. The hull laminate must be entirely dry. Sealing moisture inside guarantees the blister returns next season. Leave the craters exposed to the air for at least a few weeks. Months are better.
If you are working in a damp British boatyard over winter, you must help the process along. Tape a skirt of polythene around the hull. Place a small dehumidifier or a low-wattage tube heater underneath. Use a marine moisture meter like a Tramex Skipper Plus. You want the reading inside the crater to match a known dry area of the hull well above the waterline.
Prime and fill craters
Once dry, wipe the craters with a clean rag dampened with acetone to remove surface oils. Brush on a coat of unthickened epoxy resin. West System 105 resin with 205 fast hardener is the industry standard and costs around £50 for a 1.2kg pack. The liquid epoxy penetrates the exposed glass strands and seals the bare laminate.