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