And yes, they might be aware of a few exceptions such as Frans Hals. But they might not realize that, by the late 1700s, several important painters were not making totally solid paintings. That was because subjects of portraits looked crisp and carefully done (this is what viewers mostly focused on -- faces, etc.). What tends to be ignored are other parts of the same painting that were not painted with the same exactitude.
This post considers Thomas Gainsborough (1727-1788) and his painting Portrait of Anne, Countess of Chesterfield (1777–1778) that resides in the Getty in Los Angeles.
Biographical information on Gainsborough can be found here, and the Getty's web page on the painting is here.
Let's take a look.