I spent a few years working with glassfibre , and I think the best thing to use for filling cracks as you describe would be Clear gel coat, with catalyst ( hardener ) and your chosen powder , brass .copper . aluminium , etc all mixed to a thick'ish paste for bigger cracks, and thinner for narrow cracks , you can seal one side of the crack with sellotape to prevent it running through and going to waste , with small quantities great care will be needed to get the correct ratio of catalyst to gel . Try to fill the cracks as flush to the surface as possible and this should reduce the need for excessive sanding , or as some one else has already mentioned , before it fully hardens , when it is a firm toffee consistency it can be trimmed with a sharp blade flush to the surface of the crack , and it should sand easily . CFS ( Redruth ) should understand if you chat to them .