Method for treating a fibre stock for making of paper, board or the like and product

A method for treating a fibre stock for making of paper or board, the method comprising

• obtaining a fibre thick stock,
• adding, to the fibre thick stock, at least one cationic first agent, selected from the group consisting of:
• cationic starch having a charge density of 0.1-2 meq/g;
• cationic copolymers of acrylamide or methacrylamide, having a charge density of 0.2-5 meq/q;
• and any of their mixture;
• and
• adding separately to the fibre stock and after adding the at least one cationic first agent, at least one anionic second agent, which is an water-soluble anionic copolymer of acrylamide, methacrylamide or acrylonitrile, and which has an anionic charge of 0.4-5 meq/q in such amount that a ratio of an added absolute cationic charge to an added absolute anionic charge is from 1:0.1 to 1:0.95,
• whereby the cationic first agent is allowed to interact with the fibres in the thick stock before the anionic second agent is added, and
• whereby adding the at least one cationic first agent increases original zeta potential value of the fibre stock to a first zeta potential value, which is in a range of −15-+10 mV, and adding the at least one anionic second agent decreases an obtained first zeta potential value by 1.5-10 mV.