When it comes to aircraft like this in the image that have been converted from passenger to cargo aircraft, do they plug the windows with metal or do they just paint over them?

Passenger-to-freighter (P2F) aircraft conversions address window openings through structural plugging rather than cosmetic painting, a distinction with significant engineering and regulatory implications. When an airframe originally certified as a passenger transport undergoes conversion to an all-cargo configuration, the window apertures in the fuselage skin must be treated as structural elements, not aesthetic ones. Approved conversion programs — such as those offered by Boeing Converted Freighters (BCF), Elbe Flugzeugwerke (EFW), ST Engineering, and Precision Conversions — use aluminum or composite plug assemblies that are mechanically fastened into the existing window frames, restoring the hoop-stress load path through that section of the fuselage. These plugs are typically flush-riveted and finished to match the surrounding skin, making them nearly invisible on a well-painted aircraft but readily identifiable on unpainted or bare-metal fuselages as slightly different-toned circular or oval patches.

The reason paint alone is never an acceptable solution comes down to pressurization and structural certification. A commercial transport fuselage operates as a pressure vessel, and each window cutout represents an interruption in that vessel's skin. Passenger windows are engineered with multi-pane glazing, retention systems, and fail-safe framing to manage that interruption. Removing the glazing without installing a certified structural plug would leave an unpressurizable void — or worse, a point of fatigue crack initiation — that no airworthiness authority would approve. The plugs must meet the same fatigue life and damage tolerance standards as the surrounding structure, and their installation is governed by a Supplemental Type Certificate (STC) that defines the approved materials, fastener patterns, and inspection intervals.

From an operational standpoint, the treatment of window plugs matters to cargo operators and their maintenance organizations because it affects recurring inspection requirements. Depending on the STC, plug installations may require periodic eddy current or visual checks for fatigue cracking at the fastener holes, particularly on high-cycle narrowbody airframes like the 737-700/800 and 757-200 that are increasingly entering the converted freighter market. Operators flying these aircraft under Part 121 or Part 135 must ensure their approved aircraft flight manuals and maintenance programs reflect the specific conversion STC, since window plug condition feeds directly into continued airworthiness determinations during heavy maintenance visits.

The broader context for this question sits within a major surge in P2F conversion activity that has been underway since the mid-2010s and accelerated sharply during the e-commerce boom of the early 2020s. Boeing 737s, 757s, 767s, and Airbus A321s are all active conversion platforms, with backlogs at conversion shops stretching months to years. The visual signature of window plugs — whether painted body-color or left as bare aluminum patches — has become one of the most recognizable marks of the modern cargo fleet, particularly on the narrowbodies now dominating last-mile and regional freight networks. For pilots transitioning from passenger to cargo operations, understanding that these plugs are certified structural components, not cosmetic repairs, reinforces a broader point about cargo airframe airworthiness: the freighter may look like a stripped-down transport, but every modification on it carries its own STC, its own inspection program, and its own engineering pedigree.