A hydrophobically sized fibrous web layer, preparation of a fibrous web or a fiber-based coating, a multilayer board product having at least a middle layer formed of said fibrous web, as well as use of a heat-sensitive surfactant for said methods and products, whereby microfibrillated cellulose (MFC) and hydrophobic size are brought to a foam with water and the heat-sensitive surfactant, the foam is supplied to a forming fabric of a paper or board machine, dewatered by suction of air through the forming fabric, and dried to a web product. Alternatively the foam may be supplied onto a premade fibrous web and dried to form a coating layer. The hydrophilic functionality of the surfactant contained in the web may be destroyed by heating. Pulp of a greater fiber length, such as CTMP, may be included, to provide improved wet and dry tensile strength for the paper and board products.