We bought a gelding that you could not spray with anything! I started out just spraying him when he was eating hay. I would let him move about and spray some more then when he got where he did not mind that I worked up towards the head I would cover his eye so he could not see the spray bottle coming near his neck or ear area.
It really helped He eventually could be sprayed anywhere and could care less.
He was the same way about water but would go into panic mode. So I started spraying him when he was in the pasture eating again during the hot part of the day. Would even have someone hold him and I start at the feet and work my way up. He got to where he did not mind at all. If you start at their feet it helps allot. Oppose to just spraying their bodies first.
Good Luck and have patience.