We print the signs on LASERTAG 305 stock wich is 100% synthetic and is completely waterproof.  The smooth surface has a laser topcoat with UV inhibitors and print enhancers for fabulous, long-lasting images.

It seems that some people have a hard time understanding water proof, so at trade shows we have a fish bowl in our booth.  We'll print cards at the boot and then put them straight into the fish bowl to demonstrate the cards are 100% waterproof straight from the printer.