Possible Clues:

• Humour shown over Arp's first method of painting
• Agency worker has lots of time for painting technique
• It’s used by artists in moderate amounts, initially
• Paint — moderate source of art?
• Alternative to oils or watercolor
• Moderate a sort of emulsion for painting
• Method of drawing water down middle of drain
• Modify a painting technique
• Type of water-based paint
• Water-based paint