MIXING THE PAINT

Paint must be thoroughly stirred to obtain uniform pigment distribution. When pressure feed tanks are used, they should be equipped with continuous air­operated stirrers to prevent pigment settling which can cause high pigment volume concentration in the paint film and lead to failure of the paint. It is important to use the proper type and amount of thinner. Too much thinner can reduce the durability of the paint. The paint suppliers' instructions for mixing should be followed.

For spray application, the viscosity of the paint should be controlled on each batch of paint after mixing and before spraying. Manufacturers' recommendations may vary slightly but are usually based on measurement with a No. 4 Ford viscosity cup. Viscosity cups are available from most chemical supply houses. Viscosity is affected by temperature and should be adjusted for a standard temperature of 73°F. The acrylic based paints are usually sprayed at a viscosity of 15 to 17 seconds as measured by a No. 4 Ford cup. Other viscosity cups may be used, but the time will be different. The corresponding time on a No. 1 Zahn cup, for example, is 24 to 26 seconds.

The color fastness of the paint film depends on the quality of the paint. I n some cases, however, poor color stability will result from mixing or reducing two normally stable colors. Before making production parts, therefore, consult the paint manufacturer for an opinion on the color stability of specific mixtures of paints.

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