How do u get a air tank as air tight as posible?

i just put a 5 gallian air tank for my air horn in my truck. it was at 150 psi at 7am and at 20 psi at 3pm so thare is quite a leak. it takes a long time for my compresser to pump it back up so i waunt to fix it. i used thred tape on all the fittings 6 turnes about the 1/2in prots and 3 times around the 1/4 witch is what the instictians that came with the tank sed to do but is thare a better way to get it more air tight