The Parceling

After a rope has been wormed — the grooves between strands filled with small line — it is parceled: strips of tarred canvas are wrapped spirally around the rope, each turn overlapping the last, laid with the lay of the rope so that water runs off the overlaps rather than under them. Parceling is the moisture barrier. It sits between the worming beneath (which fills voids) and the serving above (which provides abrasion resistance). The parceling does one thing: keep water away from the rope's fibers.

Vapor barriers in building construction serve the same role. Between the interior wall finish (drywall, plaster) and the insulation sits a sheet of polyethylene or foil-faced paper. Its only purpose is to prevent warm, moist interior air from reaching the cold insulation, where it would condense and cause rot and mold. The vapor barrier is the parceling of the wall assembly: it does not insulate, does not finish, does not support. It prevents moisture from reaching the layer that moisture would destroy.

Conformal coating on electronic circuit boards is parceling at the scale of components. A thin layer of acrylic, silicone, or urethane is applied over the assembled board, encapsulating solder joints, traces, and component leads. The coating does not improve the circuit's performance. It prevents moisture, dust, and chemical contaminants from reaching the conductive surfaces where they would cause corrosion and short circuits. The board works the same whether coated or not — until the environment attacks.

Parceling is the layer that addresses a single environmental threat without contributing to the system's primary function. The rope's strength comes from its fibers. The wall's thermal performance comes from the insulation. The circuit's function comes from the components. The parceling, the vapor barrier, and the conformal coating do nothing for these primary functions. They prevent the one environmental agent — moisture — that would degrade the primary function if it reached the working layer. Parceling is protection so specific that it looks like nothing until the day the thing it protects gets wet.