Have you tried a clear epoxy laminating resin with a fast hardener. You have to use a fast hardener or the resin drains back off the vertical surfaces of your mold.
 Start your layup when the gel layer will still leave a fingerprint but will not stick to your finger.
 You may have to do some research on this to find a resin system that is compatible with your layup resin.
  Clear urethane varnishes have also been used with good results, however they are known to eat through PVA mold release film. One trick used to stop this is to spray over the PVA with Cellulose nitrate Clear dope. This is removed from the part with Acetone after the part is demolded.
B.E.

Hi BE,

Thanks for your thoughts and apologies for taking so long to reply. I'm actually producing the part with resin infusion (sorry, should have mentioned that), so am limited to using epoxy infusion resins rather than hand-lay up resins.
I've since found and tried a Polyurethane lacquer by Becker Acroma that works well as an initial 'gel-coat' - very clear and dries pretty fast and hard. I'm using a TR semi-permanent release product which does a good job, but do occassionally use PVA, so will bear your advice in mind! Certainly don't need any demoulding issues to compound problems!