I'll assume it's a NPT fitting. Clean it as best you can with brake Clean, let it dry good ( or heat it with a torch) then use Pipe Dope. Messy crap but if the fitting is clean it works.
FYI,
- Reliability: NPT pipe threads have always been popular, as leaks can be fixed by simply tightening the fitting. This practice, however, has led to cracked fittings. While JIC has been the industry standard for years, it still susceptible to cracked flares from overaggressive wrenching. The mated sealing surfaces in O-ring face seal fittings and DIN metric fittings reduce the risk of over torqueing, and provide greater reliability due to the elastomeric seal that is captured in the mating surface.
At the end of the day, all
hydraulic fittings are only as good as the assembler who installed them. A general rule of thumb is to connect a hose with the least amount of connections possible. There are proper assembly procedures for each of the fitting styles, and strict adherence to those steps makes the difference between a solid connection and a problem waiting to happen.