My bathroom ceiling is wood cladding which has been stained then
polyurethane varnished. It's a fairly dark pine colour. I want to
lighten it and have some Bathroom quality emulsion for it. What would
be a suitable base for this, after a light sanding of course,bearing in
mind the colour change? Would ordinary primer/undercoat be suitable, or
what?

Regards and thanks in advance

Syke