What causes flattening or dulling in a paint finish?

Flattening or dulling in a paint finish often occurs due to the presence of inferior thinner or wax on the surface. When low-quality thinners are used, they may not properly evaporate or may leave behind residues that can affect the clarity and gloss of the paint once it dries. This can lead to a lack of shine and an overall dull appearance. Additionally, if wax is present on the surface prior to application, it can interfere with the adhesion of the paint and cause it to not cure properly, also resulting in a flattened finish.

The other factors mentioned do not directly cause flattening in the same way. High humidity can lead to issues such as improper drying or defects, but it typically does not cause a direct dulling effect. While using old paint may lead to various application problems, it does not inherently cause the flattening of a finish. Quick drying agents, on the other hand, are intended to speed up the drying process and, if used correctly, usually do not contribute to a dull finish either.