Answer: There are two rules of thumb to follow when pruning: never remove more than one-third of the live material in any one pruning session and don't cover the pruning cuts with any material at all. The tar-like tree seal will only slow or stop the tree's ability to callous over the pruning wounds. They can also hold too much moisture against the bark and cause rot. Just make clean pruning cuts and allow them to heal on their own.
Best wishes with your tree!
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