Painting Aluminum Castings

Q. My company manufactures and paints small cast aluminum parts for interior decorative use. These parts are never touched or handled in service. However, they may be subject to cold weather temperatures in storage prior to their interior installation. To decorate these parts, we perform the following steps prior to the final decorative spray paint coat: aluminum cleaner bath, aluminum etchant, conversion coating and finally a primer coat.

How many of these customary aluminum preparatory steps can we eliminate from our process? We’ve experimented with eliminating the conversion coating and primer coat without any adverse affects over the short run. K. T.

 

A. It is always safer to adhere to a standard pretreatment method in the organic finishing process, such as the four-step process you described. This will provide you with a uniform and consistent paint base. However, if you decide you do not need this level of surface preparation for your product, since it is not handled nor touched in service, you can experiment with modifying your process.

You may be able to eliminate the conversion coating step, if you apply the primer immediately after etching before the aluminum oxidizes. You may be able to eliminate the primer if your topcoat is formulated to be applied directly to aluminum. In any event, you can confirm your short term evaluation which, according to you, indicates good results, by performing a cross-hatch adhesion test on a representative sample.

The test is performed by scribing a painted area into 100 squares, 1/16 inch on a side, to bare metal using a single edge safety razor blade (these are getting harder to find). Press Scotch brand cellophane tape over the scribed area. After a few minutes, remove it by peeling briskly. The number of squares remaining intact on the scribed area is the percent adhesion.