some people use sterilizer sold for home brew beer kits, have heard that works well. I recently refilled a 20litre port barrel that had been empty for 6-7 years. My wine maker mate simply advised me to boil water and fill the keg with very hot water (not to add immediately after boiling) Left it overnight and not only did it stop any leaks i initially had but it didnt have any mouldy odour. I then swished a bottle of Bookmark tawny through it and left it for a couple of hours, then emptied it and added my bulk port. Worked a treat.