An aqueous polymer dispersion, characterized in that said polymer comprises units and/or groups issued from: a) at least one (meth)acrylic monomer which is a (meth)acrylic ester of a C 1 -C 12 alcohol which alcohol is linear (C1-C12) or branched (C 4 -C 12), without any other functional group than (meth)acrylate b) at least one from monomer b1) or initiator b2) or both b1) and b2) as defined below: b1) at least one vinylic, allylic or (meth)acrylic monomer, preferably vinylic or (meth)acrylic monomer, bearing an anionic group derived from strong acids with pKa < 3, preferably < 2 b2) at least one initiator, bearing the same anionic group as b1) c) at least one hydroxy-functional vinylic, allylic or (meth)acrylic monomer, preferably vinylic or (meth)acrylic monomer d) at least one carboxy-functional ethylenically unsaturated monomer e) optionally, at least one vinylic monomer bearing only one vinyl group, without any other functional group than vinylic group f) optionally, at least one vinylic or (meth)acrylic monomer bearing at least one functional group selected from: nitrile, amide, acetoacetoxy, diacetone, free silane or alkoxy-blocked silane, epoxy, urea or ureïdo g) optionally, at least one multifunctional vinylic or (meth)acrylic monomer and in that said polymer bears: - the anionic groups as defined in b) in an amount of at least 0.05 meq/g, preferably from 0.07 to 0.30 meq/g (as calculated) with respect to said polymer and - hydroxy groups as defined in c) in an amount of at least 0.1 meq/g, preferably from 0.1 to 0.8 meq/g (as calculated) with respect to said polymer. a) at least one (meth)acrylic monomer which is a (meth)acrylic ester of a C 1 -C 12 alcohol which alcohol is linear (C1-C12) or branched (C 4 -C 12), without any other functional group than (meth)acrylate b) at least one from monomer b1) or initiator b2) or both b1) and b2) as defined below: b1) at least one vinylic, allylic or (meth)acrylic monomer, preferably vinylic or (meth)acrylic monomer, bearing an anionic group derived from strong acids with pKa < 3, preferably < 2 b2) at least one initiator, bearing the same anionic group as b1) b1) at least one vinylic, allylic or (meth)acrylic monomer, preferably vinylic or (meth)acrylic monomer, bearing an anionic group derived from strong acids with pKa < 3, preferably < 2 b2) at least one initiator, bearing the same anionic group as b1) c) at least one hydroxy-functional vinylic, allylic or (meth)acrylic monomer, preferably vinylic or (meth)acrylic monomer d) at least one carboxy-functional ethylenically unsaturated monomer e) optionally, at least one vinylic monomer bearing only one vinyl group, without any other functional group than vinylic group f) optionally, at least one vinylic or (meth)acrylic monomer bearing at least one functional group selected from: nitrile, amide, acetoacetoxy, diacetone, free silane or alkoxy-blocked silane, epoxy, urea or ureïdo g) optionally, at least one multifunctional vinylic or (meth)acrylic monomer and in that said polymer bears: - the anionic groups as defined in b) in an amount of at least 0.05 meq/g, preferably from 0.07 to 0.30 meq/g (as calculated) with respect to said polymer and - hydroxy groups as defined in c) in an amount of at least 0.1 meq/g, preferably from 0.1 to 0.8 meq/g (as calculated) with respect to said polymer. - the anionic groups as defined in b) in an amount of at least 0.05 meq/g, preferably from 0.07 to 0.30 meq/g (as calculated) with respect to said polymer and - hydroxy groups as defined in c) in an amount of at least 0.1 meq/g, preferably from 0.1 to 0.8 meq/g (as calculated) with respect to said polymer.