Greetings Marios:
Predicting tree failure is nearly impossible - all we can do is use experience and science to reasonably predict when failure is either highly probable, probable, or not probable.
Trees don't heal their wounds, rather they compartmentalize damaged areas via callus tissue or pitch (i.e., phenolics). I personally agree with leaving the "wound" open rather than applying a wound treatment (arbor paint) or other superficial treatments (e.g., cement, foam, sheet-metal, etc.).
Most arborists and foresters agree the best way to handle tree wounds is to simply let the tree take care of the wound naturally. However, that recommendation needs to be made on-site by a professional due to liability concerns.
