also keep in mind that before welding, you need to attach the two objects first, under the edit geometry parameter (assuming your using edit poly to model) theres a button called attach, press that then press on the mirrored model and they should be attached to one group, make sure before attaching you have the 2 models alligned together, or it will be a ***** to weld up, also keep in mind that the mirrored object is a copy not instance, for some reason it wont let you attach if the mirrored copy was an instance