What Is Osmosis?
Water seeping beneath the gelcoat and forming acidic pockets in the laminate - the hull problem everyone fears, and most often misdiagnosed.
Osmosis is the process where water molecules slowly pass through the gelcoat and combine with soluble material in the laminate, creating pressurised acidic fluid pockets. The visible sign is bubbling on the hull surface; when pierced, a vinegary-smelling liquid is the classic giveaway.
More often than not, the real damage is not the osmosis itself but the misdiagnosis: not every bubble is osmosis, and unnecessary gelcoat stripping jobs are among the costliest needless bills a boat can rack up. The table below sets out the sequence to follow instead of panicking.
What to do when you spot a bubble
Finding / step | What to do |
|---|---|
Hull bubble | Check whether it is in the paint or the gelcoat: use a nail/spatula layer test |
Layer test points to the gelcoat | Moisture reading (on a dried hull, for comparison) |
Osmosis confirmed | Urgency is rarely immediate: plan for the next haul-out window |
Single bubble | Do not rush into stripping it off; photograph it and keep an eye on it |
Blistering between paint coats is far more common than true osmosis, and it is something quite different. Diagnosis should not be made by eye alone, but by comparing moisture readings on a hull that has had time to dry out.
For a boat owner who has spotted bubbles, the order is: photograph and record it, carry out a layer test in the yard, take moisture readings, and compare over the years. For more detail, see our gelcoat and osmosis repair page.