I use this formula Bill Hueg wrote down for me and I've found that it always works.
BLENDING CREAM (1 Quart):
Mix equal portions of
- Boiled Lindseed Oil (1/2 pt.)
Smith's Cream (1/2 pt.)
1 Tbsp. Japan Dryer
Thin with mineral spirits to a heavy stain.
Bill uses this with artist oils but I usually just use it with lettering enamels or bulletin enamels. It stays open and blendable but then dries nicely overnight because of the Japan Dryer.
I also use it when I want to marbelize an area. You can make transparent glazes with it and blend them with a soft, dry badger-hair blending brush. I then put down some veins using a striping sword (rather than a goose feather.) While the glaze is workable I blend the veins a bit too. Really adds to the depth and the believablity of the "marble".
ON GLASS:
I've used the blending formula with similar results on glass. Apply a layer of the blending solution to the glass first. It takes some experimentation to know how long the paint will remain open so try a sample first. I will sometimes put down a base color and then "stomp" other colors through the base color with a soft fitch or a sponge. You can also apply your color and then swirl other colors into the base using your fingers (wear latex gloves.) This produces a beautiful effect but you need to be careful not to overwork it or it will turn muddy rather than having colors trail through it.
I understand that you can also use "Jones Cream" the same as Smiths Cream, but I'm not sure who the manufacturer is.
Maybe some others can add their experience to the knowledge base here.