I could go onto a long explanation of the healing process, but I'll keep it short.
When the horse is first injured, all ointments, sprays, etc should be kept away from the wound. The only thing that is really acceptable to use at this point is water and if you absolutely need to put something on it, use a hydra-gel. The wound needs to weep. As long as what is weeping is clear, white or slightly yellow you are doing great. These are white blood cells. Make sure you are cleaning the wound once or twice a day and watching for signs of infection. If there is a significant risk of infection or other contamination, the area can be wrapped and the horse put on antibiotics. Usually the "LITHA" method works wonderfully though. (LITHA - Leave It The Hell Alone).
Once the wound has dried up, all the swelling is basically gone and there is skin on the area is when a lanolin based product can be used to keep the skin moist and supple. Hair will usually grow back and if it doesn't there's not too much you can do. Proper wound care usually reduces/eliminates any hair loss just by nature of properly healing the wound.