I use either Bullseye Shelf Primer or Primo Primer Kiln Wash depending on the project. Each is applied differently.
Bullseye or Primo?
Why Bullseye or Primo? Well, I'm glad that you asked! The two products behave very differently. Primo goes on very wet and with puddles, which lends itself to a nice smooth surface. I use Primo when I don't want texture on the glass surface that has been in contact with the kiln shelf. It won't be perfect, but it'll be pretty smooth. Primo is fire once and re-apply. It cleans up easily, but you have to clean the shelf after every use.
Bullseye is a bit easier to mix, store, and apply. If you want a smooth kiln shelf you can always smooth it out after it dries with your hand or an old wadded up pair of pantyhose. I use Bullseye when I plan on cold working the surface, or when I'm applying 20 coats in preparation for a screen melt or pot melt. Many glass artists will apply kiln wash to an already-fired shelf and keep on trucking, not /scraping cleaning the shelves between firings.
Let's talk safety before we get started.
Kiln wash in its powder form is bad for your lungs, eyes, and is very drying to the skin. I always wear a respirator, gloves, and goggles when removing old kiln wash, mixing kiln wash, and when I'm applying it to the shelf.
Bullseye shelf primer
When I use the Bullseye shelf primer I follow their instructions for mixing and applying the kiln wash. I mix 1 part kiln wash powder to 5 parts water in an old mason jar with a lid and mix well. I apply it with a Haike brush and dry in between coats with a hair dryer. If it's sunny and warm outside, I'll set the shelves up in the sun and let the sun warm and dry in between coats (as in this video).
I also apply in a different direction with each coat. I start in the corner and follow this pattern:
1 - ↑, 2 - →, 3 -↓, 4 - ← and 5 - /, allowing each coat to dry to the touch.