I'm very keen to have a crack at a GR in the not too distant future and a post on TDPRI about model planes got me thinking about a natural finish.
Back in the day when I were but a pup and building control line planes we use nitrocellulose dope ( clear and coloured ) on both the tissue and balsa which gave a very fine and glassy surface when finished with care.
Time to check out the model shops and experiment a bit, if they still carry it it may be a decent option.