Wound Dressing Research
Don't spoil your proper pruning job by coating the wounds with tree paint or wound dressing. After over 13 years of wood dressing research, Dr. Shigo and Dr. Shortle concluded that these paints do nothing to prevent decay and little or nothing to promote wound closure.
Some of them do reduce wood discoloration, but discoloration is not necessarily equal to decay. Wound dressings tested included bituminous and asphalt based paints (typical commercial tree paints), orange shellac, rosin acid and styrene butadiene combinations, sucrose (sugar) and even aspirin. Insect entry is not prevented by tree wound paints. Insects will still bore into dying wood above and below the wound-dressed area. Treating with wound dressings is mainly cosmetic in effect. Although information of this nature has been published since the 1930's, it has been largely ignored until recently.
To quote from Dr. Shigo, "If pruning cuts are made properly, there is no need for a dressing. If cuts are made improperly, dressings will not help." [Journal of Arboriculture, 9(12): December, 1983.] A proper cut takes no longer to make than an improper cut, but the consequence for the tree can be significant.
I have heard of a chemical that stimulates fast closure, however, it keeps closing and closing and closing. Undesirable results follow.