6061 is actually not the best pick for a mirror finish, it tends to get that cloudy look once the surface starts to oxidize. The shine you got is real, but it fades fast unless you clear coat it or keep it waxed. 7075 or even 2024 will hold a polish way longer if that mirror look is what you're after. So crank your feeds on those and see how it goes, but don't expect the same result from every alloy.