Re: what is lack of cross penetration lack of inter-run fusion : what is lack of inter-run fusion and lack of cross penetration in semi auto GTAW , what is the difference between it and cold lap and lack of side wall fushion . . Sorry, I first posted this under the wrong subject. However, the main difference between inter-run fusuion, lack of cross penetration, cold lap and side wall fusion is "location in the weld". Lack of cross pen occurs when welding the root internally and the hot pass externally. If you do not get adequate penetration with your hot pass, you will get lack of cross penetration. Cold lap, side wall and inter-run fusion occur from inadequate heat input or improper welding technique.