A good teaching video on spraying finishing varnish on artwork. It's by Golden but should work for any spray finish.
It seems spraying scratchboard is the bane of any artist, even with some jokingly saying to do it while taking Valium. There seem to be many opinions on which sprays are the best, but standard advice is:
spray on warm, dry days
spray in a dust free area
don't mix brands or types of spray - it can cause cloudiness and even cracking
Edited: I just bought and used Lascaux UV Protect Semi-Matte finish and I will keep buying it for now. It sprayed well, gives just a very soft sheen to the artwork, deepened the blacks nicely, and didn't show dust much at all. It is quite expensive compared to most other brands and it really has a strong toxic smell - use a respirator!
Edited again: I finally got a great finish on one - I put the board at the 45 degree angle recommended then sprayed with 5 very light coats with about 15 minutes between each. No orange peel, no dust spots, no gloppy stuff.