Finish Coat/Layer for Bench and Heater

Last summer I built the heater and bench. This summer I need to put a durable finish layer/coat on. From my research I won't have the time required (correct me here if I'm wrong) to make an earthen plaster and apply it since I'll be on-site for about 5 weeks.

Ideas I have read about include simple oil finish (does not seem durable), plaster (issue outlined above), exterior white glue or acrylic construction glue (mix with water and apply multiple coats for max penetration), using kitty litter as final layer (hey, it is clay. . . )

Has anyone tried a layer of grout? It comes in nice 25 lb. bags and a variety of colors. Description says is crack and shrink resistant.

Any suggestions and/or explantions appreciated.

Thanks.