P.
I agree with a few people's responses here. First of all, if the dog was abused, they never forget. They have a long memory for that sort of thing, and who can blame them. I have two adopted dogs and one has nipped at worker's heels and ankles, and the other one is terrified and submits. I believe that the dog is trying to protect his family and his home. But you also can't have the dog attacking people coming into your home. I would suggest keeping the dog confined in another room if you have workmen coming in. I'm not sure I agree with the "no tolerance for aggression" statement someone else has stated, because what if the dog really did need to protect you? There has to be a happy medium, and I am sure that a trainer could give you advice on that.
I know how you feel though. My one dog nipped at the pizza boy the other day. It is so mortifying!