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 Pruner's Pal allows you to spray cuts made with a 14-foot-pole pruner. A few local garden supply stores carry Pruner's Pals, but they're hard to find, so call first. You can add a little hardware to your pole pruner for less than $10, but it's awkward and you should keep plenty of extra spray can tips on hand!