How to weatherproof particleboard?

I've built a large pair of speakers to use in my equally large back yard. They will be permanently installed on masonry pedestals. I used particle board because I came upon a large quantity for free. This stuff will soak up water like a sponge and turn into mush. I'm thinking of rolling on polyester "fiberglass" resin but if anyone knows of anything that might stand up to the elements better I'm interested. Cost and time are considerations - I've thought of coating them with the aforementioned resin or poly varnish and then covering them as one would a shed, with tar paper and siding but quite frankly that's too much work. Perhaps "boat people" will know of some other finish that stands up the the elements better than fiberglass resin.