There are a number of ways of applying kiln wash/ shelf primer/ batt wash - all are separators so the glass does not stick to the shelf - to achieve a smooth surface. Remember though there will always be some texture because of the particle size of the wash. So for the smoothest surface, use the finest powder you can find.
It also is important to prepare the mixture some hours before application to ensure all the particles of the powder are wetted. Immediate use often leads to a gritty surface.
Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges.
Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage. There may be some brush marks.
To reduce the application marks further, you can brush or spray hot water over the still damp kiln wash. This helps to remove brush marks or the stippling that often comes from spraying.
Another way to reduce the texture after drying is to polish the kiln wash with a ball of old nylons or rub a flat piece of paper with the palm of your hand over the shelf. Remove the dust that may be left behind.