i have an hardwood threashold at my back door. (6" x 4', oak, stained). it gets lots of afternoon sun, and rain. what type of protection should i put on it? it is 2 yrs old, i originally used an exterior poly, but that seems to be deteriorating and the wood is getting water on it. should i use a different poly, or a marine varnish, or what?

also, can i just sand off the original poly, restain the wood, and put the new finish on?