You can seal the antler measures inside with any acrylic varnish, if you want. Most don't seal the insides, letting the porous bone alone. Instead, they use wax and buffers on the outsides to make a nice shiney, " wet" look finish. Shoe polishes are used to color the outsides of the antlers. You can spray an acrylic varnish on the outside of the antler, to hold the colors fast, and give it some protection from handling.
Most antler measures and other tools are left "as is" so that the oils and dirt from your hands produces a natural " patina" over time. It gives the measure "character" that is uniquely your own.
This is all up to your own taste and choice. There are NO RULES about this. A powder measure was a functioning tool for a particular gun. Nothing more and nothing less. If it cracked, chipped or broke, the owner of the gun simply made a new one. They were NOT art objects back in the 18th and 19th centuries. Today, we tend to get a bit GOOFY over anything hand made, and made well, so that common tools become ART. Whatever trips your trigger....... :hatsoff: