The practice of having a specialist drapery painter work on the clothes of sitters after the artist had painted in the face was quite common in 18th-century portrait painting. Rouquet offers the following observations and anecdotes.
When a portrait painter happens to have a little business, it is usual for him to employ other hands in the painting of the drapery. Two rival artists took it into their heads to hire entirely to themselves another painter whose name was Vanhaken, to be employed in the drawing of the drapery: this man had real abilities, and might have done much better things, but chose to confine himself to this branch, because he was always sure of business. The two painters agreed to pay him eight hundred guineas a year, whether they could find work for him to this amount or not; and he on his side engaged to paint no drapery but for them. When either of those painters was employed to draw a picture, it was frequently on condition that the drapery should be done by Vanhaken. And indeed his drapery was charming, in an excellent taste, and extremely natural. The two rival painters who had thus engrossed Vanhaken, occasioned a great deal of confusion among the rest of their brother artists, who could not do without his assistance. The best of them knew not how to draw a hand, a coat or ground; they were obliged to learn it, and of course to work harder. Sad misfortune! From that time ceased that extraordinary sight at Vanhaken’s, when he used to have canvases sent him from different parts of London, and by the stage coaches from the most remote towns in England, on which one or more masks were painted, and at the bottom of which the painter who sent them took care to add the description of the figures, whether large or small, which he was to give them. Nothing can be more ridiculous than this custom, which would have still continued, had Vanhaken still continued. (Rouquet 1755, 44-45).
Rouquet’s Vanhaken was Joseph Van Aken (c.1699—1749) (his recent death occasioning the ` had Vanhaken still continued’ comment) who was indeed a notable drapery painter, although I am not sure about the story of two artists cornering the market. The Oxford Dictionary of Art notes that he `worked for Highmore, Hudson, Knapton, Ramsay, and others’.