In a FRP laminate, surface mat is present under the top coat (gel coat) which creates a weather  resistant layer. Properly impregnated and cured surface mat will protect the inner glass-resin matrix. This will surely enhance life of your composite parts. At the same time, surface mat also reinforces the gel coat layer. While applying a layer of surface mat, always be generous in using resin. Ideally, glass-to-resin ratio for laminating surface mat should be at least 1:6 to a good 1:10. For instance, to lay a 30 gr/m2 surface mat, use 250 to 300 grams of resin per meter square.

Application with Glass Veil

Most important aspect is to create a barrier. In some aggressive environments where composite parts are constantly exposed / submerged into acidic waters or chemicals, a layer of surface mat is applied on both sides of such parts. One classical example is cooling tower panels.