According to the Opus art store website, "[The] Sta-Wat Painter’s Pal Palette keeps acrylic and other water-based paints moist on the open palette for hours. Once the lid is closed, the paints will stay in workable condition for days, even weeks. A unique sponge and special permeable palette paper provide the paint with a constant source of moisture."
Charla discusses the Sta-Wet Palette in the Bold School Guide. "This palette allows your acrylic paint to stay wet and workable for several weeks, our artists have even used it for more than a month."