It is known that the outer surface of a hull is covered by a special resin called gelcoat, which mainly protects it.
Thus, this gelcoat is constantly exposed to the sea-water and sun, leading to its erosion. Over the years the gelcoat loses its shine, becomes dull and porous. The bigger the pores become, the more permeable the gelcoat gets. It also stains more easily and then of course it is harder to clean it putting the hull in a great risk.