Sheaffer used their rosin based thread sealant on the section threads of most of their pens through the 70s. On Snorkels and PFMs, it serves to seal against air leaks, and also keeps the sections from turning as you propel the snorkel tube forward. Even when soft, its quite tacky.
Shellac sets too hard and can be difficult to soften enough to take the pen apart without damaging the plastic of the barrel or section. Nail polish contains solvents that can also make it nearly impossible to take apart. The rosin based thread sealant on the other hand softens at about 90F, much lower than shellac and the melting point of the plastic.