Fibreglass boats that are heavily soiled will need more than a marine shampoo, Emms Mobile Marine Valeting Service will advise what materials to use and how to use them. To protect delicate areas and seals always work top down, i.e. highest point to lowest and rinse carefully. Where there is an ingrained stain or a tough cleaning requirement a gel may be required, the professionals at Emms will be happy to advise.
Polishing and applying compound will protect the fibreglass surfaces of your boat. Using an abrasive rubbing compound will enable the removal of any minor oxidation from the gel coat and deal with scratches and another surface marking.
However, the compound is abrasive and must be done with great care and only when required to reduce the risk of permanent damage being inflicted. Various compounds, polishing materials and methods are available and professional guidance is available from Emms Mobile Marine Valeting Services.