A method for preparing a paint forming dispersion having extended shelf life, into a sealed and pressurized aerosol can, said dispersion preventing hardender compounds to be formed from hardener precursors,by bringing into said can a paint forming dispersion which contains after sealing the can: liquefied propellant, polyurethane resins of an amount, hardener precursor of said polyurethane resins of an amount and ketones; wherein free water of amount W1, which amount W1 is in the range of 1-10000 ppm, preferable in the range of 1-6000 ppm has been brought into said can alongside with said other paint dispersion forming chemicals before sealing the can; and wherein said polyurethane hardener precursors are selected from the group comprising of an imine, an enamine, a Mannich base, a Schiff's base, an aldimine, an azole preferable an oxazole such as an oxazolidine and mixtures thereof, characterized thereof, that - polyurethane resins are selected from the group which composes of MDI, HDI, IPDI, TDI and phenol blocked TDI prepolymers, wherein - the paint forming dispersion further contains a weak acid, as a catalyst, to prevent formation of the polyurethane hardeners from said polyurethane hardener precursors wherein - the weak acid, is selected from the group which have the p K a value in the range from 1.2 to 9, preferable 1.2-5.2 and the amount of said weak acid beeing in the range of 0.01 -10 % and is adjusted so, that it catalyze the reaction (2; 2a1) shifting the equilibrium of the reversible reaction (2a;2a1), in alternative pathways A or B, between the free water present in said dispersion and polyurethane hardener precursor back to formation of said initial polyurethane hardener precursor: and wherein said polyurethane hardener is a compound with at least an amine and possibly also a hydroxyl functionality and wherein said reversible reaction (2; 2a1) takes place during the entire retention time of the dispersion in the sealed can, so that the amount of free water is unchanged and remains between 1-10000 ppm, preferably 1-6000 ppm between two successive uses of the can. bringing into said can a paint forming dispersion which contains after sealing the can: liquefied propellant, polyurethane resins of an amount, hardener precursor of said polyurethane resins of an amount and ketones; wherein free water of amount W1, which amount W1 is in the range of 1-10000 ppm, preferable in the range of 1-6000 ppm has been brought into said can alongside with said other paint dispersion forming chemicals before sealing the can; and wherein said polyurethane hardener precursors are selected from the group comprising of an imine, an enamine, a Mannich base, a Schiff's base, an aldimine, an azole preferable an oxazole such as an oxazolidine and mixtures thereof, characterized thereof, that - polyurethane resins are selected from the group which composes of MDI, HDI, IPDI, TDI and phenol blocked TDI prepolymers, wherein - the paint forming dispersion further contains a weak acid, as a catalyst, to prevent formation of the polyurethane hardeners from said polyurethane hardener precursors wherein - the weak acid, is selected from the group which have the p K a value in the range from 1.2 to 9, preferable 1.2-5.2 and the amount of said weak acid beeing in the range of 0.01 -10 % and is adjusted so, that it catalyze the reaction (2; 2a1) shifting the equilibrium of the reversible reaction (2a;2a1), in alternative pathways A or B, between the free water present in said dispersion and polyurethane hardener precursor back to formation of said initial polyurethane hardener precursor: and wherein said polyurethane hardener is a compound with at least an amine and possibly also a hydroxyl functionality and wherein said reversible reaction (2; 2a1) takes place during the entire retention time of the dispersion in the sealed can, so that the amount of free water is unchanged and remains between 1-10000 ppm, preferably 1-6000 ppm between two successive uses of the can.