Abstract: Film-forming polymer prepared by polymerisation of: 5—from more than 0 up to 4 mol % of at least one ethylenically unsaturated monomer unit A selected from the carboxylic acid and sulfonic acid derivatives that can yield an acid function upon hydrolysis in water; 10—from 5 to 45 mol % of at least one monomer unit B selected from the group consisting of the N-vinyl lactam monomers of general formula CH2=CH—N—CO—R′J, the N-vinyl amides of general formula CH2=CH—NR—CO-15 R″, the monomers of general formula CH2=CR′″—COO—R″″N—CO—R′I, the monomers of general formula CH2=CR″″COO—R″″—NR′″″—CO—R″, 2-pyrrolidone-1-isoprenyl ketone, and mixtures thereof, wherein —R is H or selected from the group consisting 20 of alkyl, —R′ is a n-alkylidene radical having 2 to 8 carbon atoms, —R″ is se