I've used Permatex Super 300 brush-on liquid for many years, but it's getting very hard to find these days. Any PTFE (Teflon) brush-on pipe sealer is a good substitute when installing pipe thread fittings, but it can't be used as a general purpose gasket sealer like Super 300. NAPA still lists the stuff online, but I've had problems actually ordering it at the local NAPA store. The chocolate brown color Permatex "3-D" and "3-H" brush on sealer is not the same as Super 300, which was originally developed for use around aviation gasoline. Super300 is almost black, and does a much better job of sealing and staying pliable under heat and pressure. Putting brass pipe plugs into cast iron usually seals well without any liquid help.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!