I'm in the process of finishing up my chicken tractor.  Actually, I'm finishing the second chicken tractor - the first came to an untimely end last week, only two days after launch.  Dogs were involved; no chickens were harmed. 

After inspecting the damage, I decided that the primary problem was that I had tried to save money by using scrap wood which had been stored in a protected location outdoors for about 15 years.  I live in the Pacific Northwest, where the lifespan of an unprotected piece of wood is about ten minutes.  Apparently, even if it is stored in a dry location, the constant ambient moisture is enough to rot it out.

I made a glum trip to Home Depot, but managed to score a full set of 2x4 and 1x4 pieces of lumber from the cull bin.  Only fifty cents each!  That's my first tip for aspiring chicken tractor builders: always check the cull bin first!

Once I had the frame built, it was obvious that I was going to have to put some kind of finish on it if I wanted it to survive our climate.  I did some research online and found that wood's main enemies are water (surprise!) and UV damage.

There are a bewildering array of products you can use to protect outdoor wood.  I spent the better part of the morning researching all of them, trying to decide what would be best. 

Paint is clearly NOT an option for a chicken tractor.  For one thing, paint isn't appropriate for outdoor wood.  It traps moisture being released from the wood, which can cause blistering and create a mildew problem.  The same goes for varnish, which puts a seal over the wood.  Check out the results of this real-life test on the Wood Carving Illustrated message board:

"The "spar" varnish looked fabulous but after about 2 weeks it began to develop small cracks. In rapid order the door began to turn black, started to mold and the smell was enough to knock a buzzard off of a manure wagon."

For another thing, your chickens will peck at the paint (well, at everything).  You don't want your chickens to eat paint flakes!

The answer, then, is in a stain which will penetrate the wood.  The best recommendation I've found so far is Thompson's WaterSeal.  (The Thompson's site's own usage wizard suggests Thompson's WaterSeal Waterproofer Plus Clear Wood Protector.)  I'm sure there are others - please leave a comment if you have other suggestions!

As for applying the WaterSeal, this is an oil-based stain, so you will want to have mineral spirits on hand to clean your brushes.  Or you can do what I usually do, which is to buy those cheap sponge brushes, and just plan on throwing them away.  It's terrible, I know, but I've never had any luck cleaning brushes with mineral spirits.  I suck at this kind of thing.

Please note that this is NOT what you want to use for the inside of the chicken coop!  I have yet to find the appropriate finish for the inside, where your chickens will be spending many hours in close confines.  Chickens are very susceptible to fumes, so at this point, I'm leaving the inside of the coop unfinished.  I haven't yet gotten around to researching products for the inside of the coop, but I'll let you know what I find!  And as always, if you have any suggestions, please leave a comment!