What surface would you recommend for a mixed media painting using acrylics and watercolors?

Question

Should I just use a heavy watercolor paper?

Answer 1

A good heavy watercolor paper would work well.  Ampersand also makes a clayboard that I have heard is a unique watercolor surface.  

Answer 2

My first choice would be watercolour paper - 300lb would be best. "Art Board" (I guess you mean illustration board?) can de-laminate if you adhere something too heavy on it... the paper layers separate. Also, those boards tend to warp when used with wet media like acrylics.

Another option is to use stretched canvas, canvas panel or sealed hardboard that has been coated with a specific acrylic medium that accepts watercolour: some examples are Golden's Light Modeling Paste, Coarse Modeling Paste or Absorbent Ground.

I have used all of these for acrylic/watercolour/mixed media with great results.