A fibrous substrate material for producing a porous coating base paper or prepreg comprises a planar impregnatable structure made of cellulose fibers, which contains at least one pigment species and optionally contains further additives conventional for paper. The cellulose fibers contain a proportion of 1 to 20 wt.-% of nanofibrillated cellulose (NFC). A method for producing the fibrous substrate material comprises the steps of: providing an aqueous suspension containing a cellulose containing material and an admixture of said pigment species and, optionally, further additives conventional for paper, sheet forming, drying. The cellulose containing material contains a proportion of 1 to 20 wt.-% of NFC with a specific surface (SSA) of at least 125 m2/g.