When pruning, spray each wound right after you make the cut so none are missed. There is no need to seal old wounds because they dry out within a few days. Pruning seal is not recommended for trees other than oaks.
You can buy pruning seal at most hardware stores that carry garden supplies. A device called Reach-n-Spray by Spectracide allows you to spray cuts made with a 14-foot-pole pruner. This is now available at Home Depot and other retailers for about $20.