The invention relates to a method for mixing a compressed propellent gas and a slurry. The method is characterized in that the slurry is fed into a cavity, that the propellent gas is added to the slurry inside said cavity, that the resulting gas-added slurry is allowed to undergo a first expansion step while it is still inside said cavity, before it is allowed to exit from the cavity, and that it undergoes a second expansion step while exiting from the cavity into the ambient air, whereby it is sprayed onto a desired object. The invention also relates to an apparatus for working the method.