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This problem with matte and glossy areas has much to do with how the ground is absorbing the oil from the applied paint layer.
When we use a very absorbing ground, as gesso, and start painting with more or less diluted paints, the oil will be absorbed in the gesso .
When the paint is applied uneven on the surface ( as it always is), it will dry with uneven speed. Thick paint will take the longest time to dry.
If we paint too soon on this uneven dry surface there will typically be three different surfaces :
1. The uncovered gesso with full absorbency
2. Parts that are painted with thin and now dry paint that will not be absorbent.
3. Parts that are painted with thick paint that are not yet dry. That is where the problems will show up because the ground is still absorbent.
When we paint on a not yet dry thick layer of paint, with another layer, the top layer will normally dry first, because it need oxygen to dry. The new layer prevent the bottom layer to dry so that it will be absorbent for a long time and drink the oil from the top layer, that becomes matte.
Good craftsmanship is to either paint wet into wet or wet upon dry. And when having a wery absorbent ground, priming it to be moderately absorbent.
Sargent advised to always paint wet into wet using no or little extra medium. I have only seen one of his paintings that suffer from cracks and gliding out of the top layer. That is his late self portrait.
Allan
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