It really is true that sometimes the palette with a mixture of the day's paint caked on can appear at times to be more interesting than the work in progress. The same can be said for the glorious accidental spills and drips and strokes that appear on the under sheets or table coverings that many of us place down on our workspace before beginning a Mixed Media project. I always encourage new students to intentionally work on top of layers of paper. My favorite paper to use under work is 18 X 24 Drawing paper (which comes in gummed pads at your local art store) or rolls of white paper (Restaurant supply companies). These wonderful painted papers can be torn and used in collages or worked back into later as paintings and drawings without tearing.