I'm sorry but there is no way acclimating to fly spray should take six days...just not necessary for it to be drug out like this.
While a horse may act up a little bit on 2nd go? Most won't if handled right when teaching it/applying it.
You are teaching horse that it is something to be concerned over by dragging it out.
Show horse bottle, squeeze it a few times, use water to spray at first so don't waste money of course, and then spray horse. Work at chest and shoulder area, then sides. If horse moves, move with them, and if horse keeps it up? Get after the horse and make it stand still.
This is why having a "be still" or "stand" command is so important. Horses can and do stand still for hours.
Horse isn't terrified of it, horse has learned it can do what it wants to, hence the backing up deal.
Horse is backing up as defiance to keep from doing something it doesn't want to do. It is barn sour, spoiled, buddy sour, what have you, but at this point, it is winning the game.
It is ignoring your cues, and the whole fly spray battle is but another part of it. Horse does not respect you period.
You need to reassess your handling skills as that is the root of this more than likely. Petting, praising, letting horse walk all over you, leads to this type of thing.
Horses make me a better person.