Anyone know of a substance that I can use to coat reclaimed plywood with, that would form a smooth, water resistant surface, but that doesn't have a high ecological footprint (be it in fabrication, or in terms of aerosol delivery)?

It's for use in constructing outhouses along the design specifics of not needing toxic chemicals for purposes of cleaning, kind of a "hose 'em and forget 'em" aesthetic (I'm involved in the "Green Team" for an upcoming festival, and I've noticed that often human waste systems are where you see the heaviest environmental impact. Being able to clean an area simply with water both has the benefit of not using nasty disinfectants, but by making it easy to clean also lessens the likelyhood of people sh*tting in the forest).