In the world of pastel art, this question is frequent: "To fix or not to fix?" There are commercial sprays specially made for pastels, intended to immobilize pastel particles. This is specially desirable when the pastel is framed, because we don't want pastel particles to move and fall on the inside of the glass. When they do, the light casts a little shadow on the painting, and with the passage of time, it will become unbearable to watch, trust me, no matter how small the shadow. And the only way to correct this is to unframe the painting, clean the glass and frame again. And nobody wants this!
In my experience, the problem with these sprays is that they darken the painting or produce a slight change in colors, or worst, if the diffuser in the spray nozzle does not dispense a very fine mist, you could end up with little spray dots on your painting that will remain visible. The idea is for the fixing spray is to allow you to apply a fine film or layer of product, but sometimes this is not what you get, be it for malfunction of the nozzle of the spray can or by misuse, such as applying the spray too close to the painting. But let's say you overcome the tiny spray dots situation. For me, the other problem remains, which is the darkening of the painting or the change of color.
Por eso para mí la respuesta a «fijar o no fijar» es «no fijar». Es mi preferencia personal, pero muchos pastelistas sí fijan sus pasteles.
I don't need to use the spray because i paint over sanded pastel paper, which takes many pastel layers and grabs them very well. Plus, when i finish painting, i remove any excess pastel by hitting the borders of my painting against the ground or a table (i glue my sanded paper to the gatorfoam before i start my work). In this process, any particle that is lose, will fall from the painting. The support is rigid, as gatorfoam is very light but at the same time very strong. After all the lose particles fall, i cover my work with Duralar (similar to acetate) to protect it while on the way to the framer. Once the painting is framed, nothing moves. The key is the combination of sanded paper plus a few strokes against a flat surface to get excess pastel off. It never fails...