Modern fuels play havoc with old style taps - which can be divided into two types:
Taper plug fitting into a taper hole which uses a grease and the cork lined types. sadly all these suffer with the effect of the fuel which is now available.
The cork lined taps can be dismantled and the cork massaged with silicone grease before reassembly.
The taper plug taps must never be reseated with any cutting paste (if so they are wrecked) but can be dismantled, apply a thin smear of Red Hermetite or Hylomar Blue sealant, put the plug back into the housing in the ‘open’ position and reassemble. leave for a day to let the sealant dry - this then gives the tap taper a sealing seat.
The grease most commonly used is Molykote 3451.
It is possible that these remedies may need to done again at later intervals (depending on use). always ensure that you work safely, draining fuel out of tank before removing tap, taking necessary safety procedures whilst handling the parts and correct and careful re-assembly and refitting.