Pissarro's method would seem to avoid both the oil painter's lean-before-fat and the watercolourist's light-before-dark tropes. It sounds like a description of Pollock:
Don't paint bit by bit, but paint everything at once by placing tones everywhere, with brushstrokes of the right colour and value, while noticing what is alongside. Use small brushstrokes and try to put down your perceptions immediately. The eye should not be fixed on one spot, but should take in everything, while observing the reflections which the colours produce on their surroundings.
Cover the canvas at the first go, then work at it until you see nothing more to add... Don't proceed according to rules and principles, but paint what you observe and feel. Paint generously and unhesitatingly, for it is best not to lose the first impression.
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