Flexible, flat substrate with an abrasive surface obtained by applying an aqueous solution or dispersion of at least one pre-condensate of a thermally curable resin onto the top side and/or the bottom side of a flexible, flat substrate in an amount in the range of 0.1 to 90 wt.-% relative to the uncoated, dry substrate, cross-linking the pre-condensate and drying the treated substrate, characterized in that the aqueous solution or dispersion of at least one pre-condensate of a thermally curable resin comprises (i) a polymeric thickener selected from the group consisting of: biopolymers, associative thickeners and fully synthetic thickeners in an amount of 0.01 wt.-% to 10 wt.-% and optionally (ii) a curing agent that only catalyzes further condensation of the thermally curable resin at temperatures greater than about 60 DEG C.