copper is easier to solder, therefore making it easier to solder all of the seams
copper looks classier that aluminum
copper doesn't appear to corrode against the pot star washers, where aluminum appears to be more sensitive to bi-metallic annodization
as for shielding paint, I've seen a couple cases where the paint on an older instrument was getting flakey in the cavity. beyond the concern of the gradual loss of shielding due to this flaking, I am more concerned that these flakes will somehow find their way onto the contact plates of a control pot rendering it need of constant cleaning
My solution is to spray with clear laquer after the shielding is complete
all the best,
R