Q. We produce metallic automotive basecoats, specifically silver. We have received customer complaints about dust/dirt/aluminum pigment being present on the car body after painting, even after they filtered the material. When this happens, we bring the paint back and remix using high-speed agitation, and then re-filter the product before sending it back to the customer. This sometimes will help correct the situation, but sometimes it does not. Our quality control program for the cleanliness of our paint includes a residue test that involves filtration with the appropriate mesh during our filling and packaging process. The aluminum pigment is well-dispersed during mixing, yet dust/lumps of aluminum pigment still can be seen in the silver metallic color, either in fresh or sprayed paint. Can you advise about how this might be happening and how to overcome this?—M.F.
A. Aluminum powder has a tendency to sometimes flock or clump together during formulation, and this is especially true when certain ambient conditions are present during mixing procedures. Aluminum powder is produced in many types and sizes of flake, and is usually encapsulated with various coatings to allow for different methods of dispersion. A technique that I have found particularly effective in formulation is to pre-disperse the metallic material into a paste or slurry before adding it to the base formulation. This enables the metallic material to coalesce more efficiently into the base. Surface tension issues make it virtually impossible to incorporate dry metallic powder directly into a base, no matter how much you mix it.
It should also be noted that over-agitation, especially at high RPMs, can over-sheer the metallic particle and may break down the coating on the flake itself, which leads to a whole other set of settling and color issues. Slow, under-agitation of material also leads to improper metallic incorporation, which contributes to clumping that could be transferred during application. In addition, certain metallic paints do not perform well in cold temperatures, so it is important that both the material and the shop environment meet your application specifications. Filtration is important in capturing dirt, debris and metallic powder clumps, but the proper size filter for the flake size is crucial.
Lastly, the debris you’ve mentioned may not necessarily be in your formulation. Metallic paint and debris settle on floors, walls, etc., and can be easily transferred to a wet painted surface with air and movement around the booth. Good, sound housekeeping procedures go a long way in helping to insure this is not a contributing factor.