Wash vs. spray
Both have their uses. BN will leave less texture on the back of a piece at full-fuse temps (so will Thinfire or Papyrus). However, at slumping temperatures (i.e., in a mold), there’s less texture picked up to begin with. BN is good for texture molds where the texture from kiln wash might interfere with the design (that being said, I KW most of my texture molds). Primo kiln wash is great for texture molds — it’s quite thin and not grainy like Bullseye KW.
The advantage of a kiln wash like Bullseye is that it’s much cheaper, and in my experience, it stays on better. BN usually has to be applied before each firing (as does Primo), and you have to be careful of disturbing the BN when putting the glass in/on the mold. A kiln-washed mold can last for many, many firings.
Jester’s Baubles Fused Glass Designs