This is such an informative thread! When I took Daniel Greene's class, he had us spray retouch varnish as often each day as needed to restore values - sometimes as many as 3 times per session. You can spray retouch on a wet surface without harm, but if you use a brush-on variety, it definitely needs to be dry.
The more earth colors you use, the faster the darks seem to dry, and lighten up falsely in comparison with other pigments.
I have been using the
www.studioproducts.com Maroger ever since we took Bill Whitaker's class, Michele, and in combination with Flake White, I need to scrape my palette at noon, and set out new paints for the afternoon.
Of course this is Arizona, and you could come and hang out here with me when you have quick deadlines.
