I called this Whittle Fish a Cheat Painting at the beginning of the project. So far we have used tape to make a straight line between colors, a soda straw to make perfect fish scales, a brush handle to make the lip curve, and now I am using a fine point permanent marking pen to make the fine detail lines around the eye, the eye lashes, and to fill in the pupil.
Permanent marking pens come in all sorts of sizes from extremely thin, as shown here, to 1/2″ wide tips. Every color under the sun is available for use in your work. You can even get metallic colors, pearlized colors, and glitter pens. So, why not use them to create even, fine, perfect lines every time.