You use it just like on aluminum sheet. Scrub the aluminum with red scothbrite , apply the etching acid, rinse, and Alodine. Don't wait too long between the Alumiprep and Alodine. I plan my setup so I can finish applying the Alodine in one day.
Interesting. I made a similar comment earlier in the thread referencing the vastly different surface tensions of acid and water. That was seconded by another reader but those posts were deleted. Could say more but why bother.