Chemical to make aluminium dull quickly without long term damage?

A. Avoid the suggestion of using lye or oven cleaner [on eBay or Amazon]. Extremely few people can control the action well enough to not cause mottled finish or pitted surface.
I would look into grit blast with 400 grit aluminum oxide.
You might get by with washing it with a pressure washer [affil links] with baking soda [on eBay or Amazon] at the rate of half a pound per gallon. It is a slow process, but that is what makes it controllable.
You might be able to use naval jelly [on eBay or Amazon] , but I would sure try it on several pieces of scrap first.

A. The back end of my aluminum teardrop was way to shiny for my friend who occasionally followed me down the highway. I had great success using Easy Off oven cleaner. It turned it whitish, but I found it very satisfactory. Always try a sample area first if possible. First mask off the specific area with blue masking tape to define sanding. Then thoroughly sand the area, with the grain, with a scotch pad. Wash area off and wipe it down with acetone [on eBay or Amazon]. Remove the masking tape. Next, be sure to cover all areas you don't want to treat, using aluminum tape and 4 ml plastic. Make sure the tape is really stuck at its edge to eliminate creep. Spray on Easy Off, and let it sit 20 minutes. Then hose it off while wiping it down with a rag. Wipe dry, and remove plastic and aluminum tape. Worked great!