I wouldn't dream of painting all that bare aluminum without laying down a light coat of etching primer first.
A good etching primer from an auto body supply shop, or Duplicolor in rattle-cans (my go-to) is worth it.
After that, even house paint should stick well (but wait a month so it's fully crosslinked & cured before clawing at it).
Is zinc cromate "etching". I thought it was the supposed to be the best for aluminum. I think they made it illegal in Canada, so I figured it must be good.
i will be looking into rapco. It's amazing how many pages of people have used the Behr with good results in SS. But if I'm not satisfied on a scrap piece, it's not going on my truck.