I have had moderate success with pot metal. I would say probably 70% success. I can do stainless, aluminum and titanium just to name a few, although interestingly enough, I have not done a lot of aluminum as we don't use it alot in fine jewelry.
Some concoctions of pot metal just won't weld, as well as some forms of other metals. Seems weird, but some items just won't weld when another similar piece will. For instance, I have done several lugs on watches, where the band attaches. 9 out of ten times it welds fine, but that tenth one can just laugh at me.
Like I said, "If it can be welded, I can do it." Some just can't, but I'll sure as hell give it a try.
A door handle would be a large project as most of my welds are measured in millimeters, not inches. These welders run upwards of $35K and the welding is done under a 16x microscope, so l really could weld that bloodsuckers stinger back on...