Well, here in sunny and humid Florida, a laminated stock is almost a requirement! And yes, you should seal the areas you sand just to be safe. On the matter of material expansion - everything will change its shape somewhat under certain conditions. The trick is to control as many variables as is possible. An aluminum barrel will change differently than a steel barrel, and a fluted is different than a solid which is different than a .920 which is different that a sporter taper. I try to keep the temperature and humidity as stable as possible. Shooting outdoors try to keep in the shade, that sort of thing.