GB wrote:
Thanks for the advice guys. I will varnish the insides of the doors
to be on the safe side.

I'm still *very* unconvinced about the need to treat both sides of the
MDF the same to prevent warping; however if you're now going to paint
the insides white, then I do think that giving them a priming coat of
varnish just to match the outsides would be a bit daft...

The outsides of the doors have had two coats of varnish. If I paint
over the doors in gloss, can I just undercoat straight onto the
varnish (having given it a rub-down to provide a key)?

Sounds fine.

David