Pastel paintings can easily be spoiled by smudging. To prevent this you can spray fixative onto the paper to fix the pastel. fixative is also used when you build up a painting in layers by means of spraying and fixing each successive layer of color. Fixative comes in large and small spray cans and is sold in liquid from.
Fixative is essentially a resin dissolved in solvent. As you spray your work the solvent evaporates leaving the resin on the support. This is what holds the pastel pigment in place. When fixing an image, make sure that you pin or tape the paper to a vertical surface rather than spraying it flat;this avoids the danger of large droplets falling onto your work. Ideally, spray outdoors or in a well-ventilated but wind-free area, using a sweeping movement across the surface from the top to the bottom of your work. Be careful not to overspray your painting as the fixative is likely to alter the colors slightly, darkening them and making them lose their purity of hue.