[ynneb]

Thanks guys for your responses. I will give them all a go.

Kong the only way I can describe a mill finish is like sandblasted glass, but in this case on aluminium.
It is a uniform finish across the whole sheet and there are no discernible paths. I guess like white noise on a TV, only much finer.

I will post some pictures of the results, after I have tried each method.

EDIT: I noticed at out hardware store they are selling sandblasting attachments that fit onto a standard compressor. They only cost 20 bucks. I think I will give this a try first.