You know the problem. In a moment of over-exuberance, you’ve squeezed out too much paint on your palette and the painting is finished, leaving lots of lovely, expensive, fresh, wet paint behind. Washing it away or letting it dry seems like such a waste and not exactly environmentally friendly. Using stay-wet palettes can preserve your acrylic paint for about a week, if you keep it moist, but your next painting might not use those colours.
What to do?
I like to either paint a quick abstract, or else use the leftovers to paint a background onto some fresh canvases. Next time I come to paint, I then have a choice of plain white grounds (fresh out of the wrapper) or else I can choose a pre-painted canvas, covered in my left over paints. If I do several, I end up with a range of canvases with different backgrounds in different colours, purely down to the chance element of what paint happened to be left over.
Even the quick abstracts can make useful starting points for other paintings. An abstract background often works well with portraiture, for example.
So my advice is to buy canvases by the box and in several sizes, so that you always have somewhere to put your extra unused paint. Then you always have artistic options when you start a new work. Don’t forget that acrylic paint makes a fine background for oil painting too (but not the other way around).
There’s no such thing as waste paint. With apologies to one of my favourite comedians, Steven Wright, “I bought some used paint the other day. It was in the shape of a canvas.”
OK, OK, it was funnier when he said “house” 🙂