Why seal sections?

Back in the day, pen repairmen would usually apply a little shellac to a slip-fit section before reinserting it into the barrel. I do the same, even though some of my colleagues object to the practice.

There are multiple benefits to sealing the section in place. Perhaps most importantly, it strongly encourages the application of heat to the section-barrel joint for disassembly. Warming the barrel mouth makes it expand and become more flexible, drastically reducing the risk of breakage. When the section joint isn't sealed, there is a powerful temptation to open it up cold, whether by twisting or rocking. And though one can often get away with it, rest assured that a cracked barrel mouth is only a matter of time when opening pens cold.

This is particularly the case with sections that have a  a recessed band, as shown above, or a slight reverse taper on the part that fits into the barrel. When the barrel mouth has to be stretched in order to extract the section, doing it cold is risky indeed. 

Sealing the section also makes the section-barrel joint stronger and more secure. The pressure of writing can sometimes cause an unsealed section to rock slightly within the barrel, putting strain on the barrel while allowing the nib to wiggle annoyingly in relation to the hand. Even a very small amount of shellac will keep a section firmly in place with its shoulder resting against the end of the barrel mouth around its full circumference, preventing any rocking motion.

One final benefit: sealing the section helps keep ink out of the barrel when the pen is inadvertently dipped too deeply into the ink when filling. This is not an uncommon occurrence, and the ink that ends up trapped inside the section-barrel joint has left the celluloid there stained on more than a few older pens. We have also seen pressure bars corroded by this sort of ink exposure.

The strongest objection I can see is not to sealing sections per se, but to the use of shellac. This really doesn't apply to materials such hard rubber or acrylic, which are largely unaffected by being heated to the temperatures required to soften shellac. Celluloid, however, loses a little plasticizer every time it is heated, so there is a reasonable argument to limit the frequency and degree of heating in the interest of long-term preservation. In fact, there are other sealing compounds that require less heat to release -- most notably, various rosin-based mixes which can either be purchased or made up at home. Application of these compounds, however, is decidedly more awkward than that of shellac, since the section must be pre-warmed and the compound melted on before all parts are re-warmed and assembled.