The best time to prune maples and other trees with heavy late-winter sap flow is after they have fully leafed out in early summer. Unless you don't mind stained trunks, avoid pruning your maple tree in late winter when the majority of other trees are pruned. Maple, walnut (Juglans), birch (Betula), American hophornbeam (Ostrya), and American hornbeam (Carpinus caroliniana) have free-flowing sap that seeps from open wounds in late winter and early spring. The sap flow doesn't harm the tree, but it can create unattractive wet streaks on the bark. There is no need to seal the cuts with wound dressing. Allow nature to take its course in sealing off the cuts naturally.