Hetty I use a sheet of half inch plywood on a fiber glass table frame as my potting table and it has been serving me well for the last 12 years. I didn't put any preservative on it or paint it. What I do though, is never let anything sit directly on it when I'm not using it and after I do use it I always brush off any soil that may be on it. On the occasion I do have to leave something on it I always sit it on a wire rack so there is air circulation underneath. I even brush off leaves and pine needles regularly. There is not one speck of rot on it anywhere. As long as it can dry out completely it should last a long time. The underside actually looks brand new so I might flip it in a year or so.
You could use wood and paint it with marine varnish.
