ld want to know what the surface is your trying to clean,fibreglass gel coat,acrylic lacquer or plastic?
are you trying to remove dirt imbedded in scratches? how deep are the scratches?
all these questions need to be answered before I would touch it with anything!!
Hitting it with wet n dry could leave a cloudy finish that may attract more dirt unless its sealed.I have repaired surfboards for years and do use wet n dry to remove scratches(grade of wet and dry depends on the depth of the scratches,grading from maybe 400 down to 1200)then seal it with spray coat of acrylic lacquer,then wet n dry that followed by polishing with cutting compound.This gives you a shiny sealed finish.My 2x bobs worth