there have been a number of reports over the years on the card SD14 from Marcle and they all recommended that all you need is a good card/timber glue to glue the card model together, i.e. Aliphatic resin which is water proof, and between sections of double bottoms and double sides treat all areas with a good outdoor polyurethane varnish in satin, and liberal final coats on the outside of the hull when finished, and this gave them ample waterproofing once paint had been added to the ourside of the boat, finally finishing off with a good wax on the hull.
If you go down this route though...DO NOT FORGET, you won't be able to use cellulose or acrylic spray paint, only enamel spray, as the cellulose and acrylic would destroy the integrity of the oil based varnish and you'd end up with a very sticky orange peel mess.
neil.