g. Thinning. Thinners and solvents have definite functions in coating materials; but for
economic reasons, packaged products generally contain the smallest possible percentage of volatile
material compared to solid. The resulting high consistency makes it necessary to thin packaged
materials to varying degrees to suit the particular material, the application method, and the surface being
coated. General rules for thinners and solvents are -
Add thinner to the coating material. Adding coating material to thinner may result in an
irreversible separation of some of the coating ingredients. To ensure complete mixing,
you should add the thinner slowly to the coating material while stirring continuously.
Use only the recommended thinner indicated on the paint-can label. The wrong thinner
may destroy certain characteristics of the coating material that are not immediately
evident in the wet state.
Use caution and good judgment when thinning coating materials. Reduced material that
has increased in viscosity due to evaporation will require corrective thinning. However,
some coating materials which undergo chemical processes, such as polymerization, to
create a film cannot be thinned beyond a certain point and remain usable. After the
chemical action has progressed beyond a certain point, the coating material cannot be
brought back to the correct viscosity.
Determine the thinning required for a particular job by using good judgment and actual
testing. Excessive thinning or insufficient thinning is detrimental to the normal flow
characteristics of a coating. Excessive thinning leads to runs and sags, and insufficient
thinning causes rough coatings. The amount of thinning used for spray application
directly influences the evaporation rate of the volatiles passing from the paint gun to the
surface; a major portion of the thinner can be lost en route. The cooling effect of this
evaporation, plus the cooling due to expansion of the atomizing air, may lower the
temperature of the coating material as much as 20€F below that of the surrounding air.
The temperature of the deposited film may fall below the atmospheric dew point, and
atmospheric water vapor will condense the film. For example, when air temperature is
70€F and the relative humidity is 70 percent, the dewpoint is 63€F or 7€F below the
ambient temperature. At this point, the adhesion qualities of enamel may be reduced.
The refrigerating effect will also affect the drying and the flow of quick-drying materials.
h. Aluminum-Pigment Mixing. To overcome the difficulties encountered when dispersing
aluminum paste or powder pigments (lacquers and varnishes), follow these steps closely:
(1) Weigh and measure the amount recommended on the container.
(2) Place the weighed pigment in a clean container that is large enough to contain the entire
mix and allow room for stirring. In a separate clean container, measure the volume of the vehicle to go
with the pigment.