Professional way, Sulfuric acid bath, then alodine. Sulfuric acid strips all oxide(Powder) alodine is corrosion preventative and paint adhesion medium, rinse after sulphuric acid and the alodine, dry before painting. Scuffing with the sulphuric acid helps to prep surface. My acetic acid (vinegar) method is not recommended by the refinishers that I talked to but has worked for me. Any of the retail aluminum prep solutions that etch the surface will work fine. The professional method is used on A/C aluminum surfaces. Your rack shouldn't bring on the same stresses that and A/C experiences but hey it sure won't chip or peel. I used to bake "STEEL" car parts before painting them for the first coat. Tough as nails finish if you keep painting thin coats as it cools. NOTE: Always keep the spray can nozzle that gives you a fine even coat. Clean the nozzle with an upside can of your wife?s crappy color spray paint of the same type paint. Don't waste your favorite color. :wink: