"As Stuart was quick to read the character of a sitter, so had he a clear insight into the color of his complexion, and never was he known to fail in this particular...He commenced a portrait by drawing the head and features, and then he sketched in the general tone of the complexion; for this he seldom required more than four or five sittings, and frequently it was done in three sittings. The picture was never touched except when the sitter was in the chair. At the second sitting he introduced transparent flesh-tints, at the third he began to awaken it into life and give it expression, and then the individuality of the sitter came out. This was always done quickly."
Jane was a talented artist herself, it could be presumed she assisted her father in finishing more than a few paintings. Having assistants finish the paintings was not at all unusual. Below is an example of one which is known to have been finished by Jane.
from Lawrence Park:
Oliver H. Perry was a son of Christoper Raymond Perry of South Kensington, RI, by his wife Sarah Wallace Alexander. He became a midshipman in 1799; lieutenant in 1807, after serving in the Tripolitan War; and master-commander in 1812. In 1813 he was ordered to Lake Ontario, and while in command of the squadron on Lake Erie, he attacked the British fleet and gained a complete victory. For this service he was promoted to captain and received a medal and the thanks of Congress. In 1815 he commanded the 'Java' in Commodore Decatur's sqadron in the Mediterranean. In 1819 he sailed for the West Indies, where he died of yellow fever.
In 1818 the Rhode Island legislature commissioned Gilbert Stuart to paint a full-length portrait of naval officer. Perry sat for this portrait just before leaving for the West Indies. The full length portrait was never finished (who knows what minor altercation/irritation prevented its completion), Gilbert only completed the face, the rest was finished by 16-year-old Jane after his death.