The box that I am preparing is a small cast aluminum box that typically has a rough finish in the raw. This one is similar to the Hammond 1590B.
Every aluminum box should be sanded before painting to remove the oxidized aluminum from the surface so that the paint can adhere better to the base metal.
For this type of brushed or buffed finish, I am using an abrasive wheel that mounts in a hand drill.
The red and gray wheels are similar but the latter is a finer mesh that produces a smoother finish. The material seems to be some sort of tough nylon woven fiber much like the 3M abrasive pads that are commonly available.
I bought these buffing wheels in a kit at Harbor Freight