We've talked about pruning to the branch collar before but here is an example of what can happen when you don't.
Once that branch collar has been removed, the tree has no way to heal itself. This is now a open wound for the life of the tree which has already become horribly infected with bacteria. The oozing is the tree trying to get rid of the disease by flushing it out. But what is going to happen is the bacteria will start to break down the cell tissue and introduce rot inside the tree.
This heart rot will decay the interior of the tree which is dangerous because we can't see the extent of the decay. Trees like this will continue to grow higher but without the sound wood inside to support the structure. At some point the wind will snap the tree.
There are ways to test the tree's integrity by finding out how much sound wood is still inside the tree.
Painting the wound will not help. They should not even be selling wound dressing paint any more because university studies have proven it actually inhibits the cells ability to heal the wound.
Another tell tale sign the bacteria is rotting the interior of the tree is what is called a conk or mushroom growing out of the side of the tree. These mushroom looking growths will form when there is a lot of dead and decaying wood inside a tree. This is a sure sign you need to be suspicious of the trees structural integrity.
A tree actually can continue to stand with the interior rotted out. I have a customer with a tree that you can stand up in, (if the raccoon that likes to live there will let you). The tree is very old and the outer wood that is living is doing a good job of keeping the tree erect. However, we know that this tree is very acceptable to failure and have pruned it to make sure it is less likely to do damage.
So make sure you study the correct way to prune off a limb, leaving the branch collar.