Something that I did with my molcajete when I first got it was I sealed it. First, I ground a bunch of rice in it, until the grit stopped showing in the rice. Then washed it out and let it dry a few days. Then I took some
food grade polyurethane (water based, though not all water based are food grade), and brushed it, inside and out, and let it dry. Then I did this again, and the third time, the inside stopped soaking in, so it was totally permeated with it, and I wiped it out, and let it cure for several days, before using. I can actually wash these in the DW, as the polyurethane permeated the stone, and basically becomes a plastic layer, yet it is still coarse. I also put some silicone seal on the 3 feet - just a dab of it, then a flat piece of plexiglass, with a piece of waxed paper between them, to flatten it. Once cured, the waxed paper comes off, and that silicone keeps the feet from sliding around, and scratching the counter. That thing is well over 30 years old, and the seal is still good. You can sort of see the seal on it, in this photo.
Molcajete and granite mortar and pestle. by
pepperhead212, on Flickr