I am prepping some aluminum parts for my home built aircraft prior to taking them to a powder coating shop. I have basically sanded all of the surfaces using aluminum oxide emery cloth and finishing it off by buffing with 3M scotch bright pad. However, this process seems to leave behind an aluminum oxide residue. If I wipe the aluminum with a white paper towel, I pick up a dark gray aluminum oxide residue. What is the best method that I could use to clean the parts prior to taking them to be coated? J. A.
Congratulations! You just proved that the “white towel” test is still the best way of proving that there are inorganic soils on a part. This test has been documented many times and is the most accepted means of showing the presence of oxides on a product surface.
Now the question becomes what to do to remove this soil. Chemicals called deoxidizers will etch the surface clean of these inorganic soils. They are applied usually in a spray form and rinsed with clean water. Be sure to dry the surface before powder coating. This method of cleaning is best done at the coater’s facility just before powder application. The chemicals are available from any pretreatment chemical supplier, and may already be included in the coater’s normal cleaning and pretreatment system. They can be based on citric acid, phosphoric acid or hydrofluoric acid chemistries. They can be in a separate stage of an inline spray washer or as an additive to another stage.
This information will make you knowledgeable enough to ask the right questions to your custom coater. He should know what to do. If he doesn’t, go elsewhere for your powder coating.