Cleaning and sanding a surface to be painted is standard practice. Cleaning the surface removes dirt which can muddy your paint job, while dulling the surface promotes adhesion. Aluminium is prone to tarnish, which is difficult or impossible to remove with soap and water. According to the New Mexico State University Cooperative Extension, white vinegar can effectively clean and restore aluminium surfaces.
Boil a solution of 1 tbsp vinegar and 1 qt. water. Use more vinegar and water if necessary, but preserve the ratio.
Boil aluminium parts in the solution if possible. If your aluminium parts are too large to submerge in a pot of boiling water, generously sponge your painting surface with the solution.
Allow your aluminium to boil, or leave the vinegar and water solution on your aluminium until the tarnish disappears.
Rinse your aluminium. Repeat the vinegar treatment, if necessary.
Sand your aluminium surface in preparation for painting.