Polyester Gelcoat is used against your mould for the perfect finish that you require for your article. Applied at 0.5mm you should achieve a coating of around a 1.5sqm per kilo. Mixed at a 2% ratios with the supplied catalyst your work time will be 15-25 minutes, so you need to have your mould all set up and ready to go with it’s release agents already applied.
This is the coating on the side of a white fibreglass campervan or boat. As Gelcoat dries quite tacky, unless you are going to fibreglass over the top of it, Flowcoat is the product to use.