[lunarhighway]

i used cardboard once to make small wheel dams, i build them up useing pva (white wood glue) and covered them with a layer of this glue, next i painted them black useing acrylic paint. they held up quite well.

i think some sort of varnish might work well, perhaps a can and brush would work better then spraying as it would soak the cardboard and than dry, creating something similar to glassfiber.

i'd avoid any sort of tape. it will effectively protect the carboard, but if there's a tiny leak somewhere it will trap the water on the inside and the carboard will act as a sponge.

since you're doing a grillblock the card might not have to be all that rigid so perhaps something like a cereal box or similar that already has a gloss coat is a good starting point.

also some sort of greece could repell water although its nor that practical and will get dirty.