Note that DuPont made both
#7 White Polishing Compound and #7 Auto Polish and Cleaner in the past. Today they still make the first product, but
#7 Auto Polish is its own different brand and may be the same or an unrelated product. I can't tell, and I don't know for sure which one the OP is referring to?
In the case of the first item, above, it can be mixed with mineral spirits and dispersed in the corncob that way. If you want economy, though, just pick up a 5 lb bag of diatomaceous earth at the garden center for about the same price as a bottle of polish. It's sold as a mechanical insecticide that works by scuffing the wax coating off bugs, allowing them to dessicate, but it is harmless to people and has been used as a toothpaste polish in the past. The bag I bought says "food grade", if that's any comfort. It's apparently used as a filler. Mix a slurry with mineral spirits and dispense in the corncob.
The main reason to polish cases, for me, is so I can find them in the grass more easily than darkened cases, which camouflage themselves better. The only exception is nickel-plated cases, which reflect the color of grass with enough fidelity that they disappear even more completely than tarnished brass does. I avoid the shine preservers in rifle brass. I always fear they will affect bullet pull consistency, though I've not done an experiment to prove it one way or the other.