PROCEDURE
Ideally, when treating a wound on a resistant horse, you have an experienced helper at the head of the horse, and on the same side as you. Make contact with the horse and gently move your hand toward the wound.
If possible, lightly touch the wound and withdraw before the horse has a chance to react. If the horse reacts or withdraws, keep your hand there, and have the handler continue to apply pressure at the head until the horse shows the most subtle yield or relaxation. At that second, withdraw your hand (have your handler instantly release the pressure) and give the horse 10 seconds to relax and "soak”, soothe them with your voice.
Step by step, you will ask for more, showing the horse that cooperation means relaxation and security, and resistance means intensity and restraint. Start with what is easily possible and gradually work toward the final goal of cleaning or treating the wound adequately.
When you do begin cleaning the wound, be very gentle and cause the least pain possible. Do just a bit of gentle cleaning, and if the horse accepts, stop and let the horse soak for 10-30 seconds again before continuing.
Continue cleaning the wound (even more intensely) when the horse is trying to evade and pause instantly when the horse does the "right" thing (begins to stand quietly). This shows the horse that by not evading, they are rewarded with the desired outcome. The horse will quickly alter their behavior and become easier to treat if these rules are followed consistently.
TIPS FOR SAFETY AND SUCCESS
Here I use a painful wound to illustrate the point, but this procedure would be the same for any contact that is painful for the horse.
Break the task into its component steps. End each micro-interaction on a positive note. If the wound is becoming easier to treat, you are doing the right thing and with time, most wounds also become less painful.
If each time, the procedure is becoming more difficult to perform, you are doing something wrong, and you need to pause and reconsider your method.