Joe, Pachmeyer's have carbon steel stiffeners in them and sometimes the steel peeks its way through the rubber causing rust on the grip frame. There is also the fact that the rubber will not absorb any moisture where as wood will. This moisture lays between the rubber and metal and can also cause rust. The wood absorbs the moisture and can release it slowly.

I have never had any gun rust that wore Walnut grips. I have had two sets of Packmeyers on two guns and had rusting issues with both. Not only did the grip frame have some rust on it, the steel also scratched up the blued finish. Neither gun was ever carried, these were both target guns so I can not attribute the rust to weather or sweat. This has been my experience and because of this, no more Pachmeyer's for me. YMMV.