Mix Minwax “Dark Walnut” stain with any wood preservative such as Thompsons or Cuprinol. That’s what we originally used on the trestle that’s on the cover of the December '05 issue of GR. Since then, It’s been brushed once or twice with used motor oil.
Besides not leaving it out in nature? Not really. Planting groundcovers around it will mitigate the dirt splashing up on it from rains and waterings, but other than that, there’s not much you can do. Water will splash, dust will collect, and birds will… well, you get the idea. That’s all part of the fun of being in the great outdoors.
I use a weed sprayer filled with water to occasionally clean off the buildings after a rain, or before I take photos of them. Doesn’t prevent anything, but it does gently clean them off.
Try Kamloops Junction. They make a water tower kit, and include a resin spout casting with that. I don’t know for certain that they would sell one separately, but it can’t hurt to ask. I thought I saw Hartford Products at one point having one, too, but I could be mistaken.