I was doing some research recently and came across a beautiful image from an article that I have to share. The image perfectly captures Bouguereau's frottis, rub-in technique from a painting titled: Venus with Doves circa 1879, (Oil on canvas 32-1/8" x 18-3/8").
I have seen images of this painting before, but never an accurate image like this. It's from an article entitled, "Revolution in Paint" by Perry Hunt, which is a fascinating article and I encourage you to read it. This Bouguereau is on page 4 of that article. You can zoom in on the image since it's a PDF.
Aside from going and seeing the painting yourself, this is the best way to study technique. You can see how loose, but precise his colors and tones are. You can also see how he finished the figure first and left the background to be completed later. Even the inked line is visible outlining the doves. It definitely taught me a lot.