A process to apply a foamable water-based adhesive comprises combining, in a pressure-tight container having an outlet valve, a) a first aqueous polymer having a pH-value above 6 and containing a dissolved or dispersed salt that can liberate a gas, and b) a second aqueous polymer comprising an acid and having a pH-value lower than 5, and combined aqueous releasing the combined aqueous polymers from the pressure-tight container through the outlet valve onto a substrate, whereby gas bubbles are formed in the aqueous polymers leaving the valve, thus foaming the water-based adhesive.