No matter what alloy you're using, there is a tough but transparent oxide layer covering the entire piece. Think of it as surface rust you can see through. Complicating matters is that this oxide layer melts at a higher temperature than the base metal. Use a brush with stainless steel bristles to break up the oxide layer, and clean the whole piece with rubbing alcohol. Do this right before you begin to weld because the oxide layer re-forms quickly.