I’ve worked with Ceracolors (cold water soluble wax paint) for a while and really like the effects one can achieve. They seem to work best on an absorbent surface, such as watercolor paper/panel or surfaces prepared for watercolors.
For a gift of a painting of a special heritage tree, I got the hairbrained idea to paint on unprimed natural birch panel. Intent was to cover with some transparent, some opague washes and to leave some of the birch grain showing as well. I did all that and it came out OK.
But I learned a lesson about painting on raw wood. This is not for the faint of heart! Not sure I would ever try that again.
Here’s the result.