Alligatoring is most likely to occur when a tough coating is applied over which type of coating?

Alligatoring is a term used to describe a specific failure mode in coatings, characterized by the formation of cracks or pattern resembling the skin of an alligator. This phenomenon is heavily influenced by the properties of the underlying and top coatings.

When a tough coating is applied over a flexible coating, it can lead to alligatoring because the flexibility of the underlying coating allows for movement and expansion due to environmental factors such as temperature changes. If the topcoat is too rigid or tough, it cannot accommodate this movement, leading to stress build-up and cracking as the flexible coating shifts beneath it. This incompatibility in mechanical properties causes the tough coating to crack in a pattern resembling alligator skin.

The other types of coatings, such as oil-based, water-based, and epoxy coatings, generally do not promote alligatoring when a tough coating is applied, as they lack the same combination of flexibility and restriction that lead to the stress-related cracking observed in alligatoring. Specifically, these coatings typically have properties that either remain more stable under stress or that are already designed to work well with subsequent layers, reducing the risk of disparity between layers.