There are many ways to add texture, but what it all comes down to is the medium you are using. Sure, Georges Seurat used pointillism with paints (and created the best known works using the technique), but I, myself, would go mad if I used this technique with paints. However, pointillism, cross-hatching, and any application of little lines to make up a texture is something I find useful when using pens. For pencil work, I think it is important to use a variety of lead hardness, tortillion stumps, and whatever line strokes best reflect the texture I am emulating. If you working in Photoshop, Paint Shop Pro, Gimp or a similar program, you can make good use of certain filters and rendering effects. If using a fluid medium, such as paint or colored inks, you can use small brushes to bring in texture, stippling tools-either sponges or rigged dry brushes, or even mix in another media. For example, mixing in plaster of Paris with acrylic paints can offer some grittiness and thickness not normally seen in that medium. Overall, mixing media can add great texture. It may be interesting, if you are creating scales, to have the media you use for the scales be different than the rest of the piece. Have the scales raised up on the paper or canvas by using a medium such as oil pastel or paint that has been thickened with plaster. Itico knows a painter in the UK who incorporates tile grout into her art to add some textural variety. I hope this helps.
