This is not looking good - the photograph appears to be showing an area on the main trunk, but it's not clear whether this area is near the base of the plant or much higher up, though I suspect it's the latter (near the base). If it's high up, then remove back to healthy wood, but if this wound is towards the bottom of the tree, and the same issue is appearing elsewhere, it's not looking hopeful for the tree's healthy survival. Further, as it gets worse, there's a strong risk of it suddenly falling in high winds, so its proximity to buildings and/or people nearby is a consideration if it's quite tall.
I'm not seeing any weeping, but I do wonder if that area is soft, so you might try pushing the tip of a pen or biro into it - you should not be able to penetrate it if its healthy. Otherwise, it's probably worth posting a photo showing the whole plant so its easier to tell quite where this problem is, or give more information regarding the age of this tree, its height and spread and the location of this wound.
Wound paints are no longer recommended - they don't prevent problems occurring at a wound site, and may actually increase the likelihood of problems occurring, so don't bother to use it.