The above passage is to be found in the July 1919 issue of Parkergrams, a Parker publication for dealers. By all evidence, this was standard practice for the era -- and for many modern-day repairers, remains so. Although some now advocate omitting the shellac, there are further reasons for using it that should be borne in mind. A shellac-sealed barrel-section joint must be warmed to be opened, which greatly reduces the brittleness of the parts and the risk of breakage in disassembly and reassembly. If the joint is not threaded, shellac reduces the risk of breakage in use, as the bond evenly distributes the stress of writing pressure across the entire section-barrel contact area. Without shellac, the stress is concentrated at the top front edge of the barrel mouth. Shellac also acts as a seal, preventing ingress of ink into the section-barrel joint should the pen be dipped too deeply into the ink bottle when being filled.
ADDENDUM: Shellac is also called for when the section is a tight fit inside the cap. We recently had to re-shellac a pen where the friction between the section and the cap interior was greater than that between the section and the barrel; uncapping the pen, the section ended up detached and stuck inside the cap. This is even more of a consideration where the cap threads onto the section, not the barrel.
