[Cob] Weatherproofing a cob bench

[Terra Incognita]

Hi there,

I finished building a cob bench in Ithaca, New York this summer and wasn't quite sure how to weather proof it after applying a clay plaster. It looked great and I knew it was something of a gamble but I had to leave town and figured I'd just let it's exposure be an experiment. There's no roof over the bench and it's not realistic to build one. The rain this fall eroded patches of the plaster and I intend to spruce it up come springtime. Currently, I have a tarp over it. 

What are my options to keep this bench safe in the elements if a roof is not an option? I've seen uncovered cob benches elsewhere. Is a lime plaster my best bet or are there other options?

Thanks much,

Dirk Trachy
Ithaca Freeskool
http://www.ithacafreeskool.wordpress.com