A covering of spackle, wood glue and sand (basically cement!) will preserve them for quite some time. Don't abuse the pieces, and they last. I build pieces for a large game store, and most have seen daily use for years without much wear and tear. And this with a customer base that tosses them aroung and rarely puts them away properly. About once a year we check them and do minor repairs, but honestly the GW 40k city ruins need more repairs (pieces break off, sometimes entire walls!)