Mat or cloth over a rust hole? Fiberglass, by the makers' sheets #Shorts
Both bridge a rust hole. Mat for strength, cloth for a smoother finish, the same resin on either (3M, USC, Evercoat, from their data sheets). The whole lay-up, step by step, is the full video on this channel: "Fiberglass Resin, Mat and Cloth: How to Fix a Rust Hole".
