Waterproofing Box Cardboard with brush?

[RobRomero]

I bought 6 built small scale model ships on ebay. They arrived adhesively taped or glued to a flat base of box cardboard. Thinking that might make an interesting ready-made display I painted the cardboard in Tamiya acrylic blue before I realized that I could not rinse the cardboard in water when I needed to clean the models. What would you recommend to to make the cardboard waterproof (there is not much room between the ships so I would need to be able to BRUSH ON WITH SMALL BRUSHES).
TIA