ETFE, PE, and PP surfaces were functionalized by a procedure involving the activation of the substrates with cold, atmospheric‐pressure helium plasma, formation of surface‐bound peroxides in air, and subsequent grafting of polymer brushes from these initiators by means of free‐radical polymerization. Strategies for obtaining weak and strong polyelectrolyte brushes are presented and the applicability of the method to the different polymer substrates is evaluated. The wettability of the modified surfaces was found to be increased on all three polymers, resulting in a decrease of contact angle from 100° in the unmodified state to about 10° after modification with the most hydrophilic brushes. Examples of surfaces with strong wettability contrast or with reversibly switchable wettability are shown.