Door seal light test

The mistake most buyers make on this test

Most people check door seals with their eyes from the outside, running a finger along the rubber gasket and looking for cracks or flat spots, then call it done. That catches an obviously worn gasket, but it misses pinhole rust-through in a wall or roof seam that has nothing to do with the gasket, and it misses a gasket that looks fine but has lost its seal at one corner.

The other version of the mistake is doing the test but doing it wrong: stepping inside for two seconds, seeing nothing, and stepping back out. Your eyes need real time to adjust to full dark before a pinhole of daylight becomes visible at all.

The correct way to run it

Step inside the container with someone else outside to close both doors behind you. Stand still and let your eyes adjust for a solid thirty seconds to a minute, the same as walking from bright sun into a dark room. Once they have adjusted, turn slowly and look at the walls, the roof, and the full outline of both door seals.

Every point of daylight shows up as a bright pinprick against total black. Note where each one sits: a pinhole in a wall or roof panel is rust-through, while daylight tracing the outline of a door itself points at the gasket or the door alignment instead.

What a pass and a fail actually look like

A pass is simple: pitch black, no pinpoints, anywhere. That is the whole standard, and it takes about fifteen seconds to confirm once your eyes are adjusted.

A fail is any visible point of light, and where it shows up tells you what you are dealing with. Scattered pinholes across a wall or roof panel usually mean rust-through in more than one spot, worth pairing with a look at that panel from outside for scale rust. Light tracing the edge of a door points at the gasket or the door alignment rather than the steel itself.

Where the light test fits in a full inspection

This is one point on a twelve-point walk-around, not the whole exam. It costs nothing and takes almost no time, and it catches what a walk-around from outside genuinely cannot, so it is worth doing even on a container that has passed everything else by eye.