For what I've understood a mw weapon is at stage +3. Once it gets damaged it goes down to +2, then +1 and finally to stage 0. At stage 0 it's like a normal weapon without any heirlooms. From there it can get damaged to stages -1, -2 and -3. In the end you won't be able to use it at all before you repair it. From there you can repair by paying the repair bills, each repair will take it stage upwards.