possibly to do with cross contamination.I think i read on the instructions with my welder that you need to change the torch liner if you weld stainless steel,when you've been welding mild steel,could be just my machine and its along time since i read it plus I've only welded mild steel:S
its more than likely that most diy type migs only come with a 0.6mm liner as standard , stainless wire is normally 0.8mm for these type of welders .possibly to do with cross contamination.