Are you working on a wooden top build table? As the heat from the weld sets fire to the timber it causes contaminants in the weld by pushing the gas away. You can get round this by using a 50mm+ hole saw where the joints are then you can run straight down the joint. Hollow section or black realy needs to be weld preped with a 45' grind and it's worth taking the material back to clean steel where the earth clamp is too. If your using a small shroud or hobby type torch it might pay to take it off and clean it every 5-6 welds as it gets blocked with splatter it reduces the gas flow.
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