Why is a pattern starting to appear in my gel coat?

There is a phenomenon called ‘print through’ which can appear in high gloss gel coats over time. Dark coloured gel coats are particularly prone due to the higher temperatures as they absorb more heat from the sun. The thermal distortion temperature of a material is the temperature at which is can start to lose its inherent rigidity and become plastic. The thermal distortion temperature for ordinary polyester resins can be as low as 65 degrees centigrade. At these temperatures moulded in and applied stress can start to relieve itself resulting in minute defections in the surface. The structure behind the layers of gel coat and laminates can start to appear as witness marks in the high gloss finish.

There are three main features which can appear. Vertical and horizontal lines can show where there are bonded in bulkheads and stringers. A fine ‘weave’ pattern in the surface will result from woven rovings of glass fibre cloth used the the first lay up. Also possible on both balsa and vinyl foam cored hulls is a grid or checkered pattern appearing. Balsa and foam cores are small blocks of material usually about 1″ cubes held on a scrim. This makes them pliable and able to easily adopt the shape of the hull when laid up. The resin soaks through the cuts or kerfs between the blocks and helps to bond the two skins of the sandwich together. The grid pattern is the resin in the kerfs showing up.

These print through patterns are nearly always purely cosmetic defects. The structure of the GRP laminate is not effected. Indeed it could actually be seen as a sign of a well consolidated laminate. There is only a temporary loss of mechanical properties, usually just in the surface layer. The GRP layup will not be structurally affected on cooling. GRP components subjected to high stress loading can suffer permanent plastic deformation. In fact it is a regular practise in the moulding industry to adjust GRP mouldings by stressing them into the required shape and applying moderate saturating heat.

It requires much higher temperatures, 200°c upward, before the resins will start to return to liquid form. Volatile compounds, glycols and styrene, trapped within the resin will boil and outgas forming tiny bubbles. When the resin cools it will re-harden but the bubbles will remain and the resin on shrinking will form micro cracks. The resins on cooling can recover some mechanical properties, but the laminates will never reinstate more than 50% of their original strength, dependent on the duration of exposure. Temperatures of this level however will never be seen with natural exposure to weather.