a spraying device for applying , through an injection opening , a coating material such as anticorrosion oil onto the inner walls of a hollow body , comprising a spray gun provided with an atomizing nozzle and an interchangeable front piece connected down stream thereof . the front piece comprises an ejector barrel for introduction through said injection opening and a positioning collar rigidly attached thereto , the positioning collar . the latter being adapted to define the position of the ejector barrel during spraying . a control unit connected to the spray gun comprises means for determining the duration of each spray operation from the very moment the trigger of the spray gun is pulled .