767,
If the ugliness is fuel/
oil/water stains, and the manifold is ON the engine, the aluminum brighteners ought to work, especially if you have an assortment of old tooth brushes to help it along...hey, plain old Gunk can do a LOT of clean up.
If you REMOVE the manifold, and degrease it...try the glass beads...the color will still be the same.
If it was a polished surface...that will be gone I think...maybe you have to use "spent glass" to not DULL the surface...fresh beads may cut and dull the surface...some of the guys could offer their experience here.
I prefer a rough "flat" finish on the aluminum...BUT, if you are into a polished and shiny surface...the glass shot seems NOT to be what to do.
I had my Weiand Stealth manifold shot at a "aircraft/industrial" shot peen shop with STEEL beads...changed the bright aluminum color a little to a darker shade with a brassy tone...should have used glass beads to match the mat finish on my valve covers.
Before it was shot, I had ground off all the Weiand logos/numbers with a fluted burr in 3/8th drill...lots of masking tape so as to VERY carefully not go below the overall surface.
My rational for "steel" shoot was any grinding marks would more likely be obscured. The marks are gone but didn't realize the steel would leave a color on the aluminum.
Pete