In general my peace poles require no maintenance.
Stainless Steel Peace Poles
I use a series of stainless steel referred to as 316. It has a high nickle content and never gets the white patina that more normal stainless steels, like 304, get when in exterior environments. Usually that shows up within a year or two. I had one of mine by my front door for about ten years. It still looked as though it had been planted yesterday. Then I replaced it with a the prototype of a new design.
Limestone Peace Poles
In most installations the best thing to do for limestone is nothing. However, if you are certain that graffiti will be a problem, there is a product you can get that will help seal it. And another product (I've tested everything I could find) that you can use to remove graffiti if it happens whether or not you've sealed against it. I will be listing both of those here as soon as I get this new site in a little bit better order.
Copper needs no maintenance. You want the acid rain to fall on it and continue developing the patina.