You can use a variety of sprays and compounds. My favorite is Boron Carbide on balsa. Here is a video showing me apply it. You don't have to re-apply every time. It last for dozens of uses before you need more. Apply sparingly just to cover the balsa.
So if you wanted to go strop crazy after using you final stone, first the 1micron on balsa, then would .5micron on balsa or leather be finer than straight horse butt?
In the end are we really just spliting micons? I have a horse butt strop that has collected some 1micron diamond spray, thinking about applying .5micron boron carbide and then finishing with just the horse butt . . . thoughts???
But if you go from a 10k stone to a 1 micron strop, you will be able to remove all the scratches from the stone, then to .5 micron after that, you will be able to polish the edge clean of all the teeth. This may or may not be a good idea. I say try it and see what happens. Basically, take a peek at the Grand Unified Grit Chart, and remember that putting compounds on balsa is pretty much just making a poor man's polishing stone.
Abrasive loaded anything will be harsher than unloaded anything. So BC on balsa is going to scratch whereas smooth leather is just going to straighten and buff.
"I have a horse butt strop that has collected some 1micron diamond spray, thinking about applying .5micron boron carbide and then finishing with just the horse butt . . . thoughts???"
I don't think it is a good idea to go with a finer compound when a coarser one has been previously been applied.
I feel it makes sense to NOT take a hugh jump between the highest grit stone and the compound. Where you begin with compounds on strops like balsa after using stones is really a complex subject. Thus one could follow a 30k stone with tenth micron CBN or Poly diamond at one extreme - beyond the reach of any stone - or follow a 1k stone with a 4.5 micron CBN on Balsa.
A 1 micron particle is approximately a 16k stone, so this is perfectly reasonable to use following a 10k stone.