the vessel 12 and the tank 26 respectively this approach has several advantages for one it allows the introduction into vessel 12 of inert gases such as nitrogen or argon to prevent skinning i e the formation of a dried film or skin on the surface of the coating and to promote drying of the coating and for another it allows emptying of the vessel 12 by positive pressure from source 48 rather than by a vacuum from source 50 this reduces loss of volatiles in the slurry 18 while maintaining the slurry 18 free from contamination as shown in fig9 the vessel 12 can be filled by making the pressure at 48 smaller than that at 50 solid lines on gauges 49 51 and emptied by making the pressure at 48 greater than that at 50 dotted lines on gauges 49 51 in the foregoing embodiments the natural agitation caused by the flow of the slurry has been used to maintain its particulates in suspension another method of agitating the slurry 18 is shown in fig1 in which a plasma bag 52 is enclosed in the tank 26 as the pressure in tank 26 is increased the bag 52 is deformed from all sides thus maintaining the slurry 18 agitated during the filling and emptying of vessel 12 without allowing the pressure medium to aerate it it is.