1. Use a good-quality, quiet sprayer. Pump sprayers that must be "primed" first produce their mist in near silence.
2. Control your body language. Approach the horse at his shoulder, use the spray bottle in slow rhythmic sweeps, and respect the horse's "threat" zones (his head and hindquarters).
3. Acclimatize your horse to the noise. Fill the sprayer with water and stand outside your horse's stall while he eats. Spray at regular intervals, pointing away from your horse, until he ignores the noise in favor of food. Repeat the process inside the stall, again without actually spraying the horse. A few sessions of dinner-time spraying should reduce his terror. In fact, the association with food may even make spray sounds welcome.
4. Desensitize your horse to the spray. Once noise is no longer a problem, find the level of spray contact your horse will tolerate. Start with a gentle mist, still using plain water. Pull the trigger slowly, and aim the spray at his shoulder. If even this sensation is intolerable for your supersensitive horse, you can flick water gently at him with your fingers first. When he accepts this, reintroduce the light spray. Continue at a base comfort level until he ignores the misting, and then increase the pressure of the spray. Move to other areas of the body as your horse accepts spraying at each site and intensity. Whenever he objects, return to the previously acceptable level until he settles, and then try again.