No such thing as 'porcelain' washers unless something exotic, cars don't use them. Teflon seal what you are most likely looking for there. White plastic looking thing. Or line seals at the fitting with metal to metal seal. Nothing you can put in there to make it seal better, the metal flare must be in useable condition. The threads themselves do NOT seal (not even intended to!) and putting anything on them only screws things up worse.