I read somewhere recently that you can Impregnate card with Polyurethane resin making them both resiliant and water proof. Other than that, Waterproof PVA or Cellulose Dope are options, first using thinned down dope and then neat brushed on. Beware if using Contact adhesive to fit the plates as Cellulose will possibly soften and detach the plates!

Make a few test pieces and try the materials out that you have to hand.

Spray paint.