the spray gun shown on the drawings comprises a handle 10 , a trigger valve 11 and a pneumatic servo - motor 12 which is controlled by the valve 11 for operating a spray material valve 30 ( fig4 ). the trigger valve 11 and the servo - motor 12 are supplied with pressure air through connections on the handle 10 . at its forward end the gun is provided with an atomizing nozzle 13 which is supplied with spray material under pressure through a conduit 14 and the spray material valve 30 . the design and operation of the servo - motor 12 is described in u . s . pat . no . 3 , 559 , 891 and a brief description only will be presented below in connection with the control unit . downstream of the atomizing nozzle 13 the gun is coupled to a connection block 16 which is provided with an inlet nipple 17 for connection of a pressure air conduit . at its downstream end the connection block 16 is provided with a quick coupling means 18 . a front piece 20 is connectible to the connection block 16 by means of a quick coupling means 21 mating with that of the connection block 16 . pressure air may be supplied through nipple 17 in order to support the transportation of atomized spray material out through the front piece 20 . the front piece 20 comprises an ejector barrel 22 which is designed so as to obtain a desired spray position when introduced into a cavity through an injection opening 23 . see fig2 . as the cavity 24 has a certain shape and as the injection opening 23 has a certain location relative to the cavity it is essential that the ejector barrel 22 be brought into a correct position for obtaining a proper covering of the cavity walls . however , a correct design of the ejector barrel itself is not enough to exactly define the desired spraying position and direction . the ejector barrel 22 has to be fixed relative to the cavity as regards direction , angular position and depth of intrusion . this is obtained by a positioning collar 25 which is rigidly attached to the ejector barrel 22 . the positioning collar 25 is designed so as to put the ejector barrel 22 into the desired spraying position by engagement with the outer surface of the body in which the cavity is located . in order to obtain a correct distribution of the coating material in the cavity it is necessary that the ejector barrel 22 as well as the positioning collar 25 be individually designed and adapted to each of the injection points . the character of an injection point is defined by the shape of the cavity and the very location of the injection opening in relationship thereto . a good covering of a cavity is obtained simply by introducing the ejector barrel through an injection opening and ensuring that the positioning collar is put into alignment with the outside surface of the body forming the cavity . most cavities , however , have to be treated in two different injection points because those parts of the cavity walls that are located close to one injection opening can only be properly covered from another injection opening . the spraying device according to the invention is by its front piece 20 specially adapted to a predetermined injection point at a certain cavity . this means that this device very well may be used at a great number of similar spraying objects for instance car bodies . during car manufacturing where a great number of different cavities on each car body shall be coated it is possible either to use a number of spraying devices each provided with one front piece adapted to a certain injection point or a few spray guns in which the front pieces have to be changed for adapting them to a number of injection points . at some injection points it may be difficult to obtain a proper alignment between the positioning collar 25 and the outer surface of the body if the front piece 20 is rigid . in order to facilitate the application of the spraying device at such injection points the front piece may be provided with a flexible hose part 28 between the positioning collar 25 and the quick coupling means 21 . the servo - motor 12 of the spray gun comprises two pistons 31 , 32 attached to a common piston rod 33 . a cylindrical bore in the spray gun housing and a stationary intermediate wall 34 define together with the pistons 31 , 32 three separate cylinder chambers 35 , 36 , 37 of which chambers 35 and 37 communicate with each other through a passage in the piston rod 33 . when all three of the chambers 35 - 37 of the servo - motor 12 are pressurized the spray material valve 30 is kept closed , and as the chambers 35 and 37 are depressurized the valve 30 is open . a control unit 40 is connected to the spray gun via the conduits 41 , 42 and 43 . the last mentioned conduit communicates directly with the chambers 35 and 37 of the servo - motor 12 . the conduit 42 communicates with the chamber 36 as well as with the trigger valve 11 . the conduit 41 communicates with the trigger valve 11 only . the control unit 40 which is connected to the pressure air source 45 via a filter 44 comprises a control valve 46 which via the conduit 43 controls the pressure air supply to chambers 35 and 37 of the servo - motor 12 . the control valve 46 has the operation order not which means that it is open in its rest position . in its rest position valve 46 maintains communication between the chambers 35 and 37 and the pressure air source 45 . the control unit 40 is arranged to establish direct contact between the pressure air source 45 and the chamber 36 of the servo - motor 12 via the conduit 42 . a valve 48 having the operation order yes is arranged to connect the left hand side of the control valve 46 to the pressure air source 45 , thereby accomplishing closing of the control valve 46 . the left hand side of a sensing valve 49 of the operation order not is connected to the trigger valve 11 of the spray gun via the conduit 41 . the left hand side of the sensing valve 49 is pressurized towards its closed position as long as trigger valve 11 is not activated . when the trigger valve 11 is activated the left hand side of the sensing valve 49 is depressurized and valve 49 is shifted from its closed to its open position . the inlet of the sensing valve 49 is directly connected to the pressure air source 45 . when activated , sensing valve 49 accomplishes opening of the valve 48 . between the valves 48 and 49 there is connected an alternating valve 51 , which is described below , and a valve 52 of the operation order not . between valve 52 and the alternating valve 51 there is a timing means 53 comprising a variable restriction valve 55 and a volume 56 . the operation of the described spraying device is as follows . as the control unit 40 is connected to the pressure air source 45 pressure air will reach cylinder chamber 36 of the servo - motor 12 via the conduit 42 . thereby , the chamber 36 is pressurized . at the same time pressure air reaches the control valve 46 which is pressurized towards its left position as shown in fig4 . in this position valve 46 lets pressure air through to the conduit 43 and cylinder chambers 35 and 37 of the servo - motor 12 . the pressure load on the right motor piston 32 is equal in both directions so the pressure load acting upon the left piston 31 in the chamber 35 will be determining for obtaining a closing force upon the spray material valve 30 . at the same time pressure air reaches trigger valve 11 which in its inactive position ( see fig4 ) allows air to be conducted to the left hand side of the sensing valve 49 via the conduit 41 . the right hand side as well as the inlet of valve 49 are directly connected to the pressure air source , but due to the fact that the cross section of the right hand side of the valve is less than the cross section of the left hand side the valve is maintained in its closed right hand side position . the pressure air source also communicates with the right hand side of the valve 48 which has the operation order yes . because of the fact that the left hand side of valve 48 is not pressurized the valve is maintained in its left , closed position . as trigger valve 11 is activated the pressure supply to the left hand side of the sensing valve 49 is discontinued , which means that this valve is shifted to its left hand , open position . now pressure air may pass on to the alternating valve 51 which in turn allows air to pass to the inlet as well as to the right hand side of the valve 52 . the latter is thereby biased toward its left open position . pressure air may now enter the left hand side of the valve 48 , and since the left hand side of the valve has a larger cross section than the right hand side the valve 48 is shifted to its right , open position . the result of this is that pressure air reaches the left hand side of the control valve 46 . due to the difference in cross section between the right and the left hand sides of the valve 46 the latter is shifted to its right hand position . in this position valve 46 connects the chambers 35 and 37 of the servo - motor 12 to the atmosphere . the constant pressure in the chamber 36 actuates piston 32 , piston rod 33 and the spray material valve 30 to the right . as the spraying device via the conduit 14 is continuously connected to pressure source for spray material , opening of the spray material valve 30 means starting of a spraying operation . as pressure air is passed through the sensing valve 49 and alternating valve 51 , air is supplied to the timing means 53 . thereby , pressure air passes the restriction valve 55 and enters the volume 56 . since volume 56 is closed the pressure therein will increase successively until full pressure is reached . by full pressure is meant the pressure of the pressure air source . as full pressure is reached the pressure load upon the left hand side of the valve 52 will be dominating and valve 52 is shifted to its closed position . now the left hand side of the valve 48 is depressurized and valve 48 is shifted to the left , thereby venting the left hand side of the control valve 46 . the control valve 46 is shifted to its left hand position and air communication is reestablished between the chambers 35 and 37 of the servo - motor 12 and the pressure air source 45 . the servo - motor 12 activates the spray material valve 30 to its closed position and the spraying operation is interrupted . the duration of the spraying operation is determined by the time interval which is needed to build up the pressure in volume 56 through the restriction valve 55 . a desired time interval for the spraying operation may be set by adjusting the cross section of the restriction valve 55 . the shown and described spraying device is fully automatic and ensures a determined length of time for the spraying operation and thereby a desired amount of spray material each time the trigger valve 11 is activated , no matter for how long this activation is continued . if for instance the trigger valve 11 is activated for a too short time interval the result would be that the sensing valve 49 shuts off the air supplied to the alternating valve 51 , the valve 52 and the timing means 53 . owing to the fact that pressure air has forced the valve 48 to its open position already the alternating valve 51 will change position so that instead the valve 52 and the timing means 53 are supplied with pressure air from a point downstream of the valve 48 . the result is that the spraying operation is continued until full pressure is obtained in the volume 56 in the way described above . in case the trigger valve 11 is activated for too long a time interval the spraying operation is still discontinued as soon as the pressure in the volume 56 is strong enough to close the valve 52 and thereby discontinue pressurizing of the left hand side of the valve 48 . the valve 48 is closed and the pressure continuously acting on the right hand side of the control valve 46 will move the latter to its open position . moreover , the described spraying device is provided with a safety valve 60 by which the conduits 41 and 42 may be interconnected . this is obtained by turning the knob 61 . the safety valve 60 is intended to ensure an uninterrupted pressurizing of the left hand side of the sensing valve 49 which means that the spray gun is not activatable . a corresponding type of valve for a manually operated spray gun is described in detail in the above mentioned u . s . pat . no . 3 , 559 , 891 . the embodiments of the invention are not limited to the shown and described example but can be freely varied within the scope of the invention as it is defined in the claims .