| Osmosis is blistering of a fibreglass hull caused by | | | | mould the hulls layer by layer, allowing each layer |
| water absorption. Osmosis is caused by long | | | | to set before putting on the next. |
| periods in the water, warm tropical freshwaters | | | | The one exception to this process is vacuum |
| are the worst culprit. Treatments involve grinding | | | | bagging or resin infusion moulding. This is where |
| out the blisters and filling with epoxy. | | | | dry layers of glass cloth and mat are laid in a |
| What is Osmosis? | | | | mould, and held in place by either an inflated |
| Osmosis is the condition of visible blisters on a | | | | plastic balloon, or another close-matching mould. |
| boat's hull, caused by water absorption. | | | | The resin is then drawn into the fibres by vacuum |
| The process of osmosis is the same mechanism | | | | suction. |
| by which plants and trees draw water up from | | | | The process is more expensive, but is capable of |
| the soil to their branches and leaves. | | | | producing very light, strong laminates with a high |
| Main Causes of Osmosis. | | | | glass content, and most importantly, very few air |
| Long periods afloat without lay-ups. Warm tropical | | | | bubbles. |
| waters. Fresh water is worse than salt water. | | | | Types of Resin. |
| Resins that are coloured (including white ones) are | | | | The earliest resins were orthopthalic polyester |
| more affected than clear resins. | | | | resins, used in the 1960s. By around 1980, |
| The Process of Osmosis. | | | | isopthalic polyester resins started to be used as |
| The GRP mouldings on the hull of a boat are not | | | | well by some builders, as they were more water |
| 100% watertight. Individual water molecules are | | | | resistant. Most builders used these more |
| so small that they can find their way into the | | | | expensive isopthalic resins only for the first outer |
| layers of glass and resin which form the boat's | | | | coats, and then saved money by using the |
| hull. When molecules of water mix with chemicals | | | | cheaper orthopthalic resins for the inner layers. |
| inside the laminate, problems start to happen. | | | | Doubling-up on the gelcoat layer is one method to |
| These are chemicals such as water-soluble | | | | reduce the risk of osmosis. |
| materials, emulsion binders for example. The | | | | Vinylester is the newest resin on the market and |
| water molecules can then have a chemical | | | | as such is the most costly, but it is even more |
| reaction with these substances, forming larger | | | | waterproof than isopthalic polyester. |
| molecules of a new chemical, often acidic. Unlike | | | | Treatments for Osmosis. |
| the original small water molecules, these cannot | | | | 1) The best option in most cases is to cut or |
| pass through fibreglass and become trapped. This | | | | grind open individual blisters, repeatedly wash out |
| is the point at which osmosis starts. | | | | with hot water or steam, to remove the 'blister |
| Waterproof Fibreglass Mouldings. | | | | juice' from any blisters, dry thoroughly and fill with |
| GRP (glass reinforced plastic), or fibreglass hulls | | | | epoxy paste. |
| are comprised of thin strands of glass. On their | | | | 2) On older boats, if there are no blisters, and |
| own the glass strands are strong but flexible. | | | | even if a moisture meter shows high readings, |
| When fused with resin it results in a more durable | | | | one option is to do nothing! If there are blisters |
| substance. Ideally the final moulding should have | | | | but they are small, they are not likely to have |
| lots of glass and only enough resin to hold it | | | | any significant effect on the structural strength. |
| together and seal in the glass strands, but most | | | | 3) Go to your local 'Osmosis Treatment Centre' |
| conventional mouldings have much more resin | | | | and have the gelcoat removed, the hull washed |
| than this. Generally, the outside of a fibreglass hull | | | | and dried out, and the hull recoated with epoxy. |
| is coated in a resin called a gelcoat. This forms the | | | | This is a costly process and therefore should only |
| hard outer surface of the hull. Most yacht builders | | | | be used if all else fails. |