I spritz Bt onto undrawn plastic frames after I add a thick layer of additional wax to the cell wall bases. I also spritz it onto any idle comb, heck, I sometimes even spritz some onto all the bees in the hive (one comb surface at a time), so the bees can spread it to everywhere they go inside the hive.
Maybe wax moths here in the desert are less picky than other places, but here, they will eat any beeswax they find, even stacks of foundation. I've also tried to keep pretty little pieces of new comb, the ones that look like a small honeycomb lollipop without the stem. They're pure beeswax from fresh Spring wax, never been used for anything - the wax moth larvae munch them into a pile of wax moth poo - and that's even inside my house. I have to keep them sealed inside a ziploc bag or bam, they're poo. Or, if I spritz them with Bt, they last much longer without being bagged.
I have the best success with Bt when I spritz a nice heavy coating onto idle combs, let them dry, then keep the combs safe and dry in stacked supers, outdoors, with covers on to keep them dry, and the Bt from possibly being washed or diluted by contact with any rain. It isn't effective forever, it does need to be reapplied (probably every other year), but it can reduce wax moth damage to just a few short web trails, or less, instead of a solid mass of webbing, cocoons, and wax moth larvae poo.
And, contrary to popular belief, wax moth larvae will eat pure beeswax, maybe they prefer a richer meal containing pollen, old bee brood castings and cocoons, etc., but they will eat just plain, pure, beeswax.
Another thing they do fairly regularly, here in the desert, any untreated brood combs are frequently affected by a few wax moth larvae burrowing through cells containing sealed brood. They don't seem to devour living bee pupae, but these affected areas are most noticeable once the affected young bees open their cappings and attempt to emerge from their cells, whence they discover that the wax moth webbing has trapped them in their cells, and they are never going to successfully emerge (they are entombed, alive). This is my main motivation to use Bt generously, especially on comb as it is being built, because this happens most often on new comb with its very first batch of brood. It can take quite a while for the bees to remove the entombed young bees and repair the damaged comb.