Hey, Paddy. For me, it depends considerably on what I intend to do next. First, I always apply pastels to a matt finish, brush off the excess with a stiff brush then rub them in with a cotton swab. If I'm pre-shading, sometimes I'll simply spray on the top coat color right over the pastels. If I'm going to apply a water color wash, I'll spray on a semi-gloss clear coat to fix them. I like to use semi-gloss because the wash won't soak in, but won't bead up and run off, either. If I'm dirtying up tires, I'll first spray the tires with a dull coat enamal then, before it's completely dry, I'll roll the tire in some loose, ground up brown/tan pastel dust and fix it with a matt finish. Like I said, depends on what I'm doing.
Cheers,
Bud