If you are removing and reinstalling any SMA-base antenna frequently, it isn't meant for that. Any seal will twist and wear, and the SMA threads themselves are easily cross-threaded and ruined. They're pretty much meant to be used once, for assembly, and left alone.
A very thin o-ring or thin butyl (non-hardening) tape can work as a perfectly good gasket seal. A light bead of E6000 ( a common "thin black rubber" type glue) will work if you're going to leave it attached. Craft stores sell "Puffy Paint" in little ounce bottles, which is a water-based plastic paint that bonds and seals well, but easily peels off when you want to remove it. And perhaps the simplest weatherproofing is a dab of silicon high dielectric "vacuum" grease. Inert electrically, but stops any water intrusion in any electrical fittings.
New antenna may be simplest, but there are so many simpler cheaper ways to ensure a good seal, available locally.