Which comes first?

The problem I had when spraying on just loose grass is that after I had the grass in the position I wanted the spray moved it somewhat. You might consider just a light coat of glue first to kind of hold it in place and then seal it with a final coat

The tried and true method for most good scenery builders is to sprinkle the various ground cover materials into the wet PAINT...let it dry and then if you feel you need more scenic materials, sprinkle it on and this time you can use adhesive to hold it. Several excellent books covering this subject in depth.

Bob

Spray a light coat to keep the grass in place, and then finish off with another spray after you have everything looking like you want it to look.  Finally, vacuum up the excess/loose grass (and repeat the processes if you find you have some bare spots).

Scenery making is very forgiving.  You can screw it up and then just go back and re-do any areas you are unhappy with, or simply cover those areas with more grass, weeds, or whatever.