so this is the natural healing process of the plant if you are worried about its health. |If it is an eyesore on your otherwise beautiful tree you can get pruning sealer which is a black thick paint (kinda) or you can use undercoating for vehicles or rino liner in a can. also pruning smaller branches will cause less stress and sap secretion so try and trim branches before they are a problem if at all possible it is also possible to heal wounds by burning or charring the wound with a blow torch but this is highly unadvisable considering how extremely flammable tree sap is also Mayonaise is excellent at removing sap and motor grease