What technics are you using to seal between the throat tube and nozzle?
A seal between the heat break and nozzle should be achieved by tightening them while the hotend is hot. After assembling the (cold) hotend, heat it up to somewhat above the highest temperature you will print at and then tighten the nozzle some more. When it cools down again, you will have a very tight seal. If the nozzle still leaks after this tightening then it is defective.
Using teflon is possible but will limit the maximum temperature you can use your hotend at safely and you won't be able to print some higher temperature materials.