Early artists like Degas used to use pastel. Back then they used to apply pastels and put a fixative on it but it wasn’t the spray we have today. They used to have to blow through a little tube, pick up the material and blow it on. After they used to have to put another coat of pastel on top and so on. This meant 3, 4 or even 5 coats of pastel to build up a picture. Today I would not apply fixative between layers. The main reason is because it does dull down the work. It would be possible to reduce this by spraying the picture then putting more pastel on top of it. I did use fixative when I was using Carb-Othello pastel pencils as the pigment wasn’t as strong. I don’t feel the need to do this now with the Faber Castell Pitt Pastel Pencils as the pigment is much stronger.
If you do need to fix your pictures then the product I recommend is “Perfix” from Daler Rowney. You can purchase it from the SAA here.