There are a number of ways of applying separators to the kiln shelf.
These go by a variety of names - kiln wash, shelf primer, batt wash, etc. - all are separators to keep the glass from sticking to the shelf. They are all combinations of alumina hydrate and china clay (or kaolin or EPK) in various amounts. The china clay provides a high temperature binder for the alumina hydrate which does not stick to glass.
These are some examples of glass separators. The Primo Primer has very little china clay, and is easy to remove. It is particularly good for small casting moulds.
The object in applying the separator is to achieve a smooth surface a possible. Remember there will always be some texture because of the particle size of the wash. For the smoothest surface, use the finest powder you can find. You can, if you want to spend the time and effort, put the powder into a rock tumbler with ceramic balls to get an even finer powder. Avoid shelf primer that is intended for ceramics, as it is coarser than that sold as a separator for glass.
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.
There are several methods for applying the kiln wash to the shelf. The two I use are spraying and brushing. Which I use depends on circumstances - spraying requires more set up time.
Spraying the separator onto the shelves can give an even coating without brush marks, runs or ridges. In this example a mould is being sprayed. To ensure an even covering on a shelf, it should be horizontal and leveled so the kiln wash is evenly distributed. Numerous light passes with the sprayer is best, as in air brushing.
Applying the kiln wash with a very soft brush such as a hake brush in a variety of directions will ensure full coverage. The brush should lightly touch the shelf and provide a number of thin layers. Applying in four directions - horizontal, vertical, and the two diagonals will ensure full even coverage. There may be some residual 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 and brushing. It is important that the shelf is perfectly level for this operation.
Another way to reduce the texture after the shelf primer dries is to lightly 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. Be sure to remove the dust that may be left behind from this polishing.
Another separator that has become popular in spite of its expense is boron nitride, often referred to by the trade name Zyp. This is a high temperature lubricant for industrial kiln operations that has been adapted for the generally lower glass forming temperatures. This is not suitable for kiln shelves, as it completely seals the porous surface of the shelf. It is difficult to go back to the cheaper kiln wash separator as the water of the kiln wash solution will not be absorbed into the shelf, leaving a patchy coverage of the kiln wash. Although both separators should be renewed after each firing (above low temperature tack fusing) the boron nitride is much more expensive and cannot provide a smoother surface than the shelf already has. My recommendation is that boron nitride use should be confined to moulds or other surfaces where the glass may slide or move in the forming process.