The present invention relates to a flexible sheet having a composite structure for stabilizing coatings, comprising a first layer of fiber material and a second layer of fiber material impregnated by a first and a second impregnating material, respectively, and a body layer set between said first layer and said second layer according to a general sandwich structure, where the body layer consists of a third material. The first and the second impregnating materials and the third material constituting the body layer are polymeric materials presenting a substantial affinity, so that the flexible sheet has a physically heterogeneous and chemically homogeneous structure.