Try using a little motor oil as a cutting fluid. That should help keep the tap sharper and make nicer threads. Also, not to add gas to the fire but you probably destroyed the tap when you tapped it through because the hole just gets tighter and tighter so the tap has to work much harder to cut the material. One other thing to try is to tap 1/2 turn then back it off 1/4 turn to break the chip, then another fresh 1/2 turn, break chip and so on. You'll get it!!