Alodine is a common Aircraft coating used on "RAW" aluminum parts that arn"t primed or painted in a traditional sense as a coating to prevent exfoliation /oxidation, It is usually used in areas that are covered, inside panel areas for example , on longerons, internal skin. Its fumes can be hazardous and direct contact with skin is not recommended. It is applied in "thin" coats and isn't a built up surface like multiple coats of paint , It is also applied over the raw surface with just an alodine prep solution wash. A better surface treatment if you plan to paint after treatment is Zinc Chromate, its trade mark is a baby crap yellow , its considered "hazardous" to spray and a good mask set up is needed. Its an MSDS product and seldom seen in the field in the scale pre 80s when we ordered it by the 55 gallon drum and sprayed it with wild abandon, Hard to get a rattle can of it now if your not in Corrosion Control AFSC. Each product has its advantages, I ve found a more acceptale solution, I spray Rustoleum , "Rusty Metal Primer " straight and allow a good drying period, generally a week in the heat. Then you can top coat it with about anything, Its a fish oil based product and has a hardy life span. Would love to just shoot Zinc Chromate and top it with a few coats of "DEATH in A CAN " two part aircraft POLY , stuff drys ROCK HARD and last forever , but oh well Good luck and great work !