Spraying is one of the most important methods for applying water-based paint. It produces a thin, even coating film, evenly distributing the paint evenly across various geometries, including those with small holes, gaps, and uneven surfaces. For spraying large surfaces, this method is faster and more effective than brushing. However, spraying does require mastering certain techniques:
Paint preparation before spraying
First, use clean water to adjust the paint to an appropriate spraying viscosity. Measure this with a Tu-4 viscometer; the ideal viscosity is generally 20 to 30 seconds. If a viscometer is unavailable, visual inspection can be used: Use a stick (iron or wooden) to stir the paint thoroughly, then lift it to a height of 20 cm and observe. If the paint does not break within a short period of time (a few seconds), it is too thick. If the paint breaks as soon as it leaves the top of the bucket, it is too thin. The ideal viscosity is when the paint is held at a height of 20 cm, forming a straight line and then instantly ceasing to drip.
Air pressure control and adjustment
Second, the air pressure should be controlled between 0.3 and 0.4 MPa (3 to 4 kgf/cm²). Too little pressure will result in poor atomization of the paint, causing pitting on the surface. Too much pressure will cause runoff and excessive paint mist, wasting material and adversely affecting the operator’s health.
The best distance between the spray gun and the workpiece
Third, the distance between the nozzle and the surface is generally between 300 mm and 400 mm. Too close will cause the paint film to run; too far will result in uneven paint mist, causing pitting. Furthermore, the nozzle’s distance from the surface will cause the paint mist to disperse and cause waste. The specific distance should be adjusted according to the type of paint, its viscosity, and the air pressure. Slow-drying paint can be sprayed from a greater distance, while fast-drying paint can be sprayed from a closer distance. The spray distance can be adjusted closer for thicker viscosity and farther for thinner viscosity. High air pressure allows for greater distance, while low pressure allows for closer distance. This “closer” and “farther” refers to a small adjustment range of 10 to 50 mm. Exceeding this range will make it difficult to achieve an ideal paint film.
Spray gun operating skills
Fourth, the spray gun can be moved up and down, left and right. It’s best to operate it at a steady speed of 10 to 12 meters per minute. The nozzle should be kept straight above the surface, minimizing slanted spraying. When spraying at the ends of the surface, quickly release the trigger to reduce paint mist. These ends often receive more than two coats of spray and are the most prone to runny spots.
Fifth, when spraying, the next coat should cover 1/3 to 1/4 of the previous coat to avoid missing paint. When spraying fast-drying paint, spray all the paint layers in one go; re-spraying often yields unsatisfactory results.
Sixth, when spraying outdoors in open areas, pay attention to wind direction (strong winds are not recommended). Operators should stand downwind to prevent paint mist from being blown onto the already sprayed paint film, creating an unsightly grainy surface.
Seventh, the spraying order is: difficult areas first, easy areas later; inside areas first, outside areas later. High areas first, low areas later; small areas first, large areas later. This will prevent paint mist from splashing onto the already sprayed paint film and damaging it.
