It's also called liquid pipe sealant, I use Loctite "PST", which is available at good hardware and DIY stores. Look in the plumbing section. It's a thick, viscous, white teflon sealant used for threaded applications where teflon tape is too difficult or awkward to use.

Apply sparingly with a toothpick and make sure you apply all around the threaded area of the nipple, re-install and lightly tighten (it's easy to strip out the threads on thin aluminum cylinders). I've had Skymaster, Robart, Jet Legend, and some unknown aiy cylinders leak until doing this.

As Keith S mentioned earlier, if the nipples don't want to come out, I apply heat with a soldering iron, it will loosen the epoxy or other (usually none) sealant used during manufacture. Ditto with removing old/hardened air line from nipples - just heat them up or melt off.

I have a set of very small ignition and jeweler's wrenches to remove and tighten fittings, but on really small ones, I uses some small, flat/wide nose pliers, being careful not to grab or mar the nipple part of the fitting.

Barry