6 years into an ongoing casual study of oil paint pigments and paint makers. Many Gamblin, Rembrandt and Utrecht pigments have clouded over with a waxy bloom. Gamblin is the worst! Old Holland stands strong with W&N in second place. No bloom or flatness; still holding natural sheen as if painted yesterday.
When painting for wet-painting sales, if you use Old Holland Paints or Winsor & Newton, there's no need to varnish your painting. The sheen of your pigments will stay fresh for years to come after the paint dries.
The worst of the pigments I tested was Gamblin's Indianthrone Blue, it's the most flat, and the waxy bloom is so opaque the pigment is about 30% lighter in value than it was the day it was painted.