Do I have to "stretch" Matthew Palmer's watercolour paper before painting on it?
No, you do not need to stretch your paper beforehand, although it may cockle slightly whilst you paint, it will dry flat. However, if this is something you are worried about you could instead try Matthew Palmer’s Watercolour Block (Code: MPWB).
The block is glued on all four sides which holds it place while you paint, then you can just peel off once it is dry.
I hope this helps,