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The goal is to remove as much dirt and grime as possible. If you are trying to clean corrosion off of your aluminum piece, the acid will need to be able to reach the corrosion to do its job. Washing as many contaminants as possible from the piece prepares it for the acid wash. A little bit of scrubbing might be needed to remove things such as burnt-on food. If this is the case, you want to use the softest method that will work. Baking soda rubbed with a rag is a good idea. Remember to use a back and forth motion rather than a circular motion to ensure an even look. Once you have washed and scrubbed the piece, it is ready for the acid wash. Rinse off any residues left behind by things such as detergent or baking soda. Dry the aluminum surface with a soft rag before exposing it to the acid solution.

Summary:
Use warm water and a degreaser to wash the aluminum surface. Choose a light abrasive for any needed scrubbing. Rinse and dry the piece thoroughly before the acid wash.