Is your 6061 alclad?

Most of my sheet aluminum use is 2024, which is almost universally
alclad.

As you probably know, 6061 is more corrosion resistant than 2024.

If the corrosion is not deep or pitted, alodine it and go on. I. e.,
use phosphoric acid treatment to remove the stain and then passivate
the surface with a chromic acid etch.