Sorted.
The object was shot against a seamless white background. All of the subject was darker than the white background.
1. In PS create new (255) white canvas, considerably larger than the image.
2. Place the image on top of the canvas.
3. Create layer mask.
4. Choose Select > Colour Range > Highlights
5. Invert.
This gave me the subject against a pure white background, with perfect white between the bristles, without having to do any brushing there.
I only had to use the brush tool to remove shadows I didn’t want. I then flattered the image and cleaned any flaws in the remaining shadow areas.
I ended up with the subject against pure white - which really sets it off - instead of against the off white of the seamless paper. More importantly, the flaws in the seamless (inevitable when moving a large object on it) were eliminated.
D.
Ps. Is it possible to set the brush tool to remove (on a layer mask) anything above a certain luminosity? If so, how? Since all parts of my subject were darker than the seamless paper, this would also work, if it’s possible to do.