MOLD!!
It can happen

Those white spots are where the mold ate through the paint.
It was an unfinished and unsealed oil painting that I never got to work on again.
It's bad to let a painting hang around for in a moist area, even if it varnished.
Mold grows on unvarnished oil paintings, it seems to be attracted to the dark Sienna's, Umber's and Ultramarine Blue.
If you live in a moist area, paint with acrylics..
Mold does not effect dry acrylics 100th as much as oil paint.
Mold can be removed with alcohol, bleach will also work. But it won't replace paint! Mold will not form in wet pigment when ammonia is added. Ammonia gas will evaporate and leave only water. Used by "T" shirt acrylic printers, egg tempera artists and fresco artists.  
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