gelcoat is a surface finish material that can be looked at as being like a thick,hard paint.it has several functions,one being a workable surface to give a smooth finish,and it waterproofs and seals the fiberglass,which can absorb water.it also keeps the fibers in the fiberglass from showing on the finish.this can happen if the fiberglass is just painted.it doesn't need to be used on all fiberglass,but is a good idea on anything that will be finished to a high gloss.