Kiln wash, or batt wash as used in the ceramics field, is largely made up of alumina hydrate, kaolin (china clay), and often some colouring to indicate an unfired shelf.
These solids are heavy and settle to the bottom of the container quickly. So, you have to agitate the contents with each dip of the brush onto the liquid. To provide adequate - and even – coverage of the shelf, mould or other refractory material, you should paint in four directions. Up, down and the diagonals. You need to apply just enough that you do not see the shelf surface.
Alternatively you can spray the solution onto the surface. This is an easier way to get an even covering, but it sometimes is overly stippled.
A tip I was given for the smoothest kiln shelf is to level the damp prepared shelf and spray a layer of warm water over the wash to form a very shallow puddle. As the water is absorbed into the shelf, the only limitation to the smoothness of the surface is the granular nature of the kiln wash.