The appearance of small circular circles on the paint film with solid particles in the center surrounded by concave circles is called "pinhole". The pinholes penetrate directly to the surface of the substance, and in fact there is still a very thin film remaining on the surface. The pinholes are generally small in size and conical in shape.
Pinhole disease is generally in varnish or enamel with relatively low pigment content, and it is easy to appear when it is applied by dipping, spraying or roller coating. The main cause is the presence of air bubbles, poor wettability to the pigment, or too thin a paint film. In addition, there are the following factors:
(1) There is water in the paint or dust in the air;
(2) Inappropriate solvent selection, too much dosage or too fast volatilization, such as asphalt paint diluted with gasoline, will be partly separated out, especially if it is baked;
(3) When diluting the paint, if the temperature is too low or the thinner is added too quickly, solvent bubbles will be formed, and pinholes will appear when the film is formed;
(4) The viscosity of the varnish is too high, and the bubbles are not easy to escape; the bubbles in the latex paint have no time to escape; the air bubbles generated during the thinning or baking process of the impregnating paint will have pinholes;
(5) Solvent remains in the primer;
(6) When the paint is mixed or the brush is too hard during construction, the air bubbles are too late to be released;
(7) When the temperature is high in summer, spray or brush the paint containing low volatile matter (such as acetone). Nitro varnish and perchlorethylene external enamel are prone to pinholes when sprayed above 30°C;
(8) When the roller coating method is applied, the roller is running too fast, or the solvent is not properly matched;
(9) Improper spraying, small nozzle and high pressure, especially the distance between the Spray Gun and the painted part is too far.
