Metal fillers generally use aluminum to deliver more flexibility, such as on a sail panel or other thin, unsupported sheet metal, and resist cracking better than standard plastic options. Finally, fiberglass fillers deliver added structure. Short-strand variations can be used to fill small holes, while long-strand formulas can cover voids of up to 1 inch in diameter. A new breed of shapable silicone putties like Sugru fill a slightly different role, molding to the shape of the parts they fix and replace. Any parts store worth its salt can point you in the right direction, and companies like 3M and Eastwood offer excellent customer support. When in doubt, pick up the phone and ask which product is right for your project.