the original intent was most likley for the copper to patinate anyway (turn green). the concept was not for a penny-bright copper statue, but a green one. if the artist (Bertholdi) wanted it to remain brown/reddish, he'd have chosen bronze with what's called a "foundry finish", achieved with chemicals, and a wax protective coating.

much damage is done these days to older statues by well-intended people with pressure washers.

bronze is a different story. when bronze truns green, it is because the foundry-applied dark bronze finish, and protective waxes, has worn away. still, it shouldn't be cleaned by abrasives or pressure washing, which will only expose fresh bronze to further weathering. the green verdigris should be removed by a conservator and the statue finished again.

copper though... copper is allowed to patinate, and at that point the oxidized finish actually protects the remaining thickness of copper from deteriorating further. cycles of cleaning and re-patination would cause damage, by exposing new copper.