Haven't had that issue, but I'd suggest think "Yay!'-time to play with spray bottles and aerosols. I prefer a rope halter, and at least 12 feet of lead, hold it about half way. Turn slightly away from the horse and spray the bottle toward the ground. If the horse needs to move his feet, let him as long as you stay safe, give him more rope if needed. Keep spraying toward the ground. The minute he stands still, stop spraying and praise and/or pet him. Rinse and repeat until he doesn't react at all to the bottle being sprayed away from him ,toward the ground. Each horse will be different on if it take a few minutes, or ???much longer. Stopping the spraying teaches him that is what you want,,,him to stand still. (Negative reinforcement--taking a stimulus away). Build from there ---get closer spray straight out, etc following the same principles...untill finally he can stand quietly while you spray him. (same scenario for just about any object that you want the horse to accept quietly. Different than MAKING the horse just TOLERATE an object by flooding) Sonny was initially anxious and couldn't stand still if I tried to have the training stick touch him or have the string swing over him. I helped him calmly accept it until I could helicopter it over his head or body and slam the string on the ground with all my strength in front of, beside, or behind him and he would just keep grazing without being upset or anxious. That reminds me....your focus and intent are very important in this -your horse reads those things and can only be calm when your focus is on the object (don't stare at him) and your intent is what you are doing with the object , NOT on making him do anything because of what you are doing with the object. Worked great for us. Good luck. No need to find an alternative way to apply liquids if you are willing to take the time to teach your horse that 'its ok'.
Basically exactly what's already been said, but in a video demonstration.
And it DOES work! 🙂