chestnut will last very well even if untreated due to all the tannins just make sure that its not cut with the sap in it during the summer else it will rot out in no time if you want treat them then there far better being partly seasoned guess you could dip them in a tank of wood preservative and leave to soak hop poles use to have the tar boiled into them theres an old tar tank in the wood that i coppice would be cool to get it going one day but doubt you could get the tar now for it