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.
The Bullseye primer dries with a pink tint so you'll know that it hasn't been fired. After firing it turns white.
Primo Primer kiln wash
Primo is a different animal. I try to mix just what I'll use at the time I need it because the powder settles quickly and can be hard to mix up again. I use the same ratio as the Bullseye: 1 part Primo Primer powder to 5 parts water.
I found a neat whisk in the kitchen supplies area at IKEA. The plastic beads at the ends of the wires are great for getting the goop at the bottom of the jar moving again.
I follow the same application pattern as the Bullseye: 1 - ↑, 2 - →, 3 -↓, 4 - ← and 5 - /, but instead of allowing the application to air dry, I use a hand-held hair dryer to gently dry it. The Primo goes on purple and fires white.
I like to prep my kiln shelves every time that I fire the kiln. It's just my personal preference. It's one potential issue that I can eliminate. Remove old kiln wash easily with one of those green Scotchbrite scrubby things, then re-coat as described above.