What is one challenge associated with using a slower drying reducer in topcoats?

The challenge associated with using a slower drying reducer in topcoats is that it can attack the substrate. This occurs because a slower drying reducer maintains a more extended period of solvent action on the surface being painted. If the reducer interacts for too long with the substrate, it can lead to issues such as softening or damaging the underlying material. This is particularly a concern with sensitive substrates, like certain plastics or untreated metals, where prolonged solvent exposure can cause warping, bubbling, or even complete breakdown of the substrate material.

Using the appropriate drying reducer is essential for ensuring a successful paint application, as the balance between working time and substrate compatibility needs to be carefully managed to avoid potential damage while still achieving the desired finish.