What does the term "burn through" refer to in coating application?

The term "burn through" specifically refers to the situation where a coating layer is sanded down to the extent that the underlying layer becomes visible. This process can occur during surface preparation or when attempting to achieve a certain level of smoothness for subsequent layers. It is critical to avoid burn-through, especially in low observable aircraft maintenance, as it can compromise the integrity of the surface, potentially exposing it to environmental factors or degrading its low observable characteristics.

In this context, the other options do not accurately reflect what "burn through" means. While applying excessive heat on materials, as mentioned in one choice, can damage coatings, it is not synonymous with burn-through. Similarly, removing coatings without precision might lead to damage, but it doesn't capture the essence of the burn-through scenario, which is specifically about sanding down to the layer beneath. Improper layering of coatings refers to techniques related to the application process rather than the act of revealing underlying surfaces. Hence, the correct understanding of burn-through is tied directly to the visibility of the underlying layer due to sanding.