Osmosis

A pigmented, resin-rich surface coat applied to the mould face before laminating, which cures to form the smooth, weather-resistant outer surface of a GRP moulding. It protects the underlying laminate from moisture and UV exposure and provides the colour and gloss of the finished part.

Chemical degradation caused by reaction with water, which breaks susceptible bonds such as the ester linkages in polyesters and some polyurethanes. Warm, humid conditions accelerate hydrolysis, which can degrade laminates, adhesives and membrane reinforcements over time.

The tendency of a material to absorb moisture from its surroundings. Moisture uptake in polymers and composites adds weight, plasticises the matrix and lowers the glass transition temperature, which is why laminates and adhesives are conditioned or dried before critical use.

A coating defect in which local loss of adhesion produces dome shaped, fluid filled or hollow raised areas in the film. Common causes include osmotic attraction of water to soluble salts trapped under the coating, retained solvent and application over contaminated surfaces.