Skin texture is a crucial part of making any portrait look realistic - digital art has always held the danger of portraits looking more like plastic dolls than people. It's not that hard to achieve if you're a little patient, and there are many brush packs to be found online with excellent brushes - these brushes were among those used in this tutorial: keepwaiting.deviantart.com/art…
. I have also had success with nature textures of sand, stone, or similar surfaces; and when you work in Painter, the Tiny Spatter and Variable Spatter Airbrushes are extremely useful. You can of course also overlay skin photo textures. Whatever works.