Thanks Steve
That sounds really good for painting something in a single colour and I'll keep a note of it for future reference. But for this project, I need a big range of different (mainly pastel) colours, so that's why I'm hoping to be able to use water colours on a matte base.
I tried an experiment today with a piece of bare wood. Gave it a coat of emulsion and when it was dry I painted a blob of water colour on top. Then after that dried I finished it off with clear nail polish (nearest thing I had to varnish!). Seems ok, so I think I'll give it a go.
Elaine