5 kg. sounds like a decent ballpark estimate. My club has an '85 DG-300 which was repainted with polyurethane last year. The original gel coat had been cut polished twice and by last year was in pretty rough shape. The shop didn't remove all the old gel coat but just lightly sanded it to take care of the oxidation and crazing. Going by the before and after weighings it gained 3.1 kg.