Finishing questions are always interesting there are a number of things going on.
The "dark" on carvings in most cases waste from mircoorganisms. I have found cleaning it with bleach works very well. The wood will get lighter after and be wet from water but after it dries I add stain, oil and wood protector or lightly re-burn things.
The surface of the carving has a lot to do with how dark things get and how fast it happens and the placement of art works. Any fibers from the saw cut left on the piece can be a place for mold and mildew to hang out. Plus a damp area will encourage it too. When it's warm and rainy and the moss comes out on the trees the little bugs are out and party they leave a dark mess like chainsaw carvers leave sawdust after the Big Buzz!
I burn my pieces with a hot torch ( oxy-propane) to create a smoother surface. I encourage my buyers to flood the wood grain with oil finish to help reduce checking which is kind of futile but helps, then use a wood protector with a fungicide and silicon like Thompsons water seal to slow or lesson the darkening. Nothing sticks silicone that's why the water beads up on the decks in the commercials.
This could be a good argument for poly's but by completey sealing the pieces there are other issues such as dry rotting and the difficulty in re-finishing works. A smooth plastic surface will not have the places for the mini bugs to hang out.
I wedge pieces for clients I often ask a fair hourly wage same thing for a re-finish. Some things only take a few minutes I say no worries and get $20 to $40 because the folks are greatful for the help.
The surface of the piece, placement of it and if they clean or maintain it at all matter. Some day I'll put up a matainance page on my web site. Looking forward to outher thoughts and ideas.
Keep your saw in the Wood!