Rust-Oleum 285093 Stops Rust Spray Paint, 12 oz, Matte Clear

I would probably go the route of an acrylic sealant spray between layers that you want to set. I have liked krylon matte sprays like this one in the past for jobs that needed a permanent layer of sealant. I have had good luck with cheaper options like this Rustoleum option. It helps to not use a gloss spray sealant for anything other than the last layer if you want a glossy finish. You want a little bit of grip for your pastels to adhere to and the matte spray can help provide that.