Method for coating parts performed in a plant wherein the plant comprises: - a conveyor for conveying a part to be coated, wherein said conveyor comprises an upper branch and a lower branch; - a coating machine spraying a coating product on a part during its conveying on said upper branch; - a cleaning unit of said lower branch comprising a reverse roller, a doctor blade, a tank, a nozzle for the removal of coating which is suitable for applying to said lower branch a cleaning fluid containing solvent and to collect in said cleaning fluid said coating product removed from said lower branch; - a reserve reservoir for the cleaning fluid containing cleaning fluid, wherein said cleaning unit (31, 32, 34, 38, 39) is connectable to said reserve reservoir to receive from it said cleaning fluid and to supply it with the cleaning fluid in which coating product was collected; - a group for treating said cleaning fluid, connectable to said reserve reservoir, wherein said treatment group receives said cleaning fluid from said reserve reservoir, to separate a pure solvent from the residues of the coating product, and to supply the treated solvent to said reserve reservoir in order to make the cleaning fluid more effective; said method characterized by the following steps: - providing a first reserve reservoir of the cleaning fluid coming from said cleaning unit (31, 32, 34, 38, 39 and optionally 31', 32', 34', 38', 39); - Recirculating said cleaning fluid between said first reserve reservoir and said cleaning unit (31, 32, 34, 38, 39 and optionally 31', 32', 34', 38', 39); - Calculating the quantity of coating product sprayed by spraying guns; when said quantity exceeds a pre-set threshold, performing the switch so said cleaning fluid is channelled to said group for treating said cleaning fluid; - Performing the treatment of saturated cleaning fluid coming from said first reserve reservoir, preferably a distillation; - Re-channelling treated solvent to said first recirculating reservoir, which is now empty; - Disposing of the solids or at worst the very viscous liquid resulting from the treatment process. - a conveyor for conveying a part to be coated, wherein said conveyor comprises an upper branch and a lower branch; - a coating machine spraying a coating product on a part during its conveying on said upper branch; - a cleaning unit of said lower branch comprising a reverse roller, a doctor blade, a tank, a nozzle for the removal of coating which is suitable for applying to said lower branch a cleaning fluid containing solvent and to collect in said cleaning fluid said coating product removed from said lower branch; - a reserve reservoir for the cleaning fluid containing cleaning fluid, wherein said cleaning unit (31, 32, 34, 38, 39) is connectable to said reserve reservoir to receive from it said cleaning fluid and to supply it with the cleaning fluid in which coating product was collected; - a group for treating said cleaning fluid, connectable to said reserve reservoir, wherein said treatment group receives said cleaning fluid from said reserve reservoir, to separate a pure solvent from the residues of the coating product, and to supply the treated solvent to said reserve reservoir in order to make the cleaning fluid more effective; - providing a first reserve reservoir of the cleaning fluid coming from said cleaning unit (31, 32, 34, 38, 39 and optionally 31', 32', 34', 38', 39); - Recirculating said cleaning fluid between said first reserve reservoir and said cleaning unit (31, 32, 34, 38, 39 and optionally 31', 32', 34', 38', 39); - Calculating the quantity of coating product sprayed by spraying guns; when said quantity exceeds a pre-set threshold, performing the switch so said cleaning fluid is channelled to said group for treating said cleaning fluid; - Performing the treatment of saturated cleaning fluid coming from said first reserve reservoir, preferably a distillation; - Re-channelling treated solvent to said first recirculating reservoir, which is now empty; - Disposing of the solids or at worst the very viscous liquid resulting from the treatment process.