Actually some of the older weapons can give the free 20%. Not all of them though. You'll need to test each of them. Good news is you don't need to be trained. Open imbuing and go to imbue spell channeling on it, but don't commit it. On the right it should say properties are 1/5 or 2/5. If it says 1/5, you have a weapon with a free 20% damage. If you have the later, it isn't worth squat. If it has old charges (like feeble, etc) on it, it may count towards spring cleaning points so check that.