I replaced the crappy stock small fuel lines with AN braided fuel lines and fittings.. Is thread seal needed? Never used on cars,,,,,,, but boats unsure???? This was from the mechanical pump to the filter to the VST.. another high design by merc. lol..
This did fix my vapor lock issues though..

No, it should seal from the flared fitting.

No thread sealer ,just a dab of Anti-sieze

If I might ask, what did you seal the pipe thread end of the fittings with ? I changed all my fuel and oil lines to AN. The sealer I used worked fine on the oil lines, but not for the fuel. Nearly every fitting leaked within 20 minutes. I even tried a Merc product from my local marina, no luck. I finally used teflon tape, stopped all the leaks.
As the other guys said, nothing needed on the flare, just don't over tighten.

I just used a little Teflon tape on the An fitting into the pump and the filter everything else was flare and had no issues last summer..Just saw some comments on sealer and though I would double check.. Thought about thread lock on the flares though as assurance for vibration

WHITE-Single density- should only be used on NPT threads up to 3/8 inch.

YELLOW- Double Density- yellow double density is often labeled as "Gas type"

RED-Triple Density: (Note-the container is red but the tape itself appears as a pale pink color). Presently required on all joints ½" diameter or greater.

GREEN- Oil Free PTFE tape- Required for use on all lines conveying oxygen (I.E. –medical oxygen or welding oxygen lines).

COPPER COLOR- contains granules of copper and is to be used as a thread lubricant but is not approved as a thread sealant. (Generally it is used as a thread lubricant on bolts or pipe threads for mechanical applications where no physical seal is required.)