There are commercial solder flow retardants, but these are NOT any of them. Here are some of the ways that goldsmiths use to prevent solder flowing.
Polishing Machine Rouge: Some people use a little rouge powder from the polishing machine dust mixed with a small amount of oil which works well. Jeff Demand of Toronto likes to make a heat the rouge until it melts and then paint it on.
Pencil Lead: a graphite stick or soft pencil will leave a barrier that solder won't flow across. Pencil leads also work to line up tube sections when constructing a hinge. Liquid graphite lubricant that can be found in for automotive supply stores can be used.
Scrap of Rubber: One can take a small piece of rubber (vulcanized mold chunk, a slice off a piece of an old torch hose etc) and, holding it in tweezers, rub it on the metal part to be protected while it is hot. The rubber melts on with a truly nasty smell (use ventilation) and leaves a brown slimy coat on the metal. This too works well.
Yellow Ochre: Old timers used yellow ochre with water. I find that it has a tendency to "bleed" into the flux and make the soldering problematic. It is somewhat better mixed with a little oil instead of water but I find it difficult to remove from the piece and do not use it.
Maalox® and "White-Out": Favorite is Papermate© correction fluid. There are solvent based and solvent-free water based versions. In my experience the water based version does not come off the metal after soldering as easily as the solvent based one. I like the solvent based one for this reason.