PTFE tape is only there to provide a seal. if after you asembled it and taken it apart, you will have to re-new the tape.
Once applied, you should start to see it lift the dirt, it will slowly start to dweel away. leave it do the work, give it 10 mins, then blast it away. obviously if the car isnt really dirty it wont make much of an impact. Iv found, if the paint work hasnt got any protection on it (wax) then it wont wok as well as if it has. as the dirt stick to the car better. instead of sticking the the wax layer.(if that makes sense?)