+1 for the large fan. I to cast in my garage and finally got around to making a casting table in front of a window and have a large box fan to suck the fumes out. Water quenching ww gets them very hard. Mine end up at 25or26 bnh. I jjust air cool almost everything my ww air cooled come out about 12 or 13 bnh which seems to be about right. The wrong hardness can cause leading also. Powder coating from what i understand should cure that.