What is it osmosis?
Osmosis is the process of a lower density liquid moving into a high-density liquid through semi-permeable membrane.
Put more simply is a natural process where two dissimilar liquids try to mix, the weaker compound trying to mix with the stronger. This effect result in a chemically induced pressure rise in the area of the high-density liquid.
What is Wicking?
Because a GRP laminate is composed of fibres bonded by a resin system, there is often a weakness in resistance to moisture flow, along the lines of the reinforcement this is known as wicking. Where a clear gelcoat has been used, the wicking will appear as a fine white fibre lines in the laminate.
Wicking can lead to serious problems unless arrested. By definition, the term ‘GRP’ means Glass Reinforced Plastic. If the fibreglass bundles are permeated by water and the water encounters on unreacted chemicals – which it will – the bundles of fibreglass will swell in a similar fashion to the osmosis induced blisters. Capillary action assists in moisture flow and clearly the fiberglass is no longer reinforcing the plastic (resin system). It is exerting the hydrostatic pressure on the laminate. This, in the limit, can result in serious degrees of disintegration, in the literal sense – the GRP is no longer an integrated compound of glass fibre and resin system but fibres of an un-bonded glass, surrounded by a degraded resin. GRP can, in serious cases be a spongy material which may be totally punctured with a steel spike. In addition, it may be porous to the extent that in the case of a marine hall, water will gradually seep from outside the boat into the bilges.
It must be added that good quality laminate, although not immune from problems or not suffered catastrophic material failure. Although, it is known that also Isophthalic polyester resins used with powder chopped strand mat (CSM), are less likely to develop problems caused by ingress of water, there are tens of thousands of marine craft, built with early formulations of orthophthalic polyester resin, which are perfectly serviceable. Early craft built in fibreglass usually have massive reserve of structural strength, by virtue of laminate thickness, although, this does not make them immune from osmotic damage. By far the most important point to be made in this section, is that this problem can exist with fibreglass structures and if there is evidence of any osmotic attack, it is very wise to have a marine hull or water container lining, inspected by a Marine Survey, who will be able to assess any current or likely future structural problems.
Where a colour gelcoat is used and this is generally the rule rather than exception, it might be necessary to remove the gelcoat for further inspection, if a moisture meter revealed high levels of moisture content. Evidence of wicking may often be seen even though a gelcoat is coloured, because the swollen fibres will read through a damp gelcoat.
Content taken from Blue Gee Glassfibre and Epoxy Wood Book