I coated mine liberally with linseed oil.

I also used linseed oil. This is what Mel recommends in his book. He also says "Its also possible to paint or stain the bottom, outside and top. Leave the inside unpainted so there's no possible contamination to the growing mix."

I'm new to this site and SFG. I just put my boxes together today and was out here looking for cheap vermiculite when I saw your post.
You might be interested in an agriculturally based oil stain called Timber Ox Green. Their site is http://www.timberoxgreen.com/index.php.
The only thing I'm not sure of is if you have to let the wood weather for a year before staining it. I heard that on some home improvement show this week. I need to call them and ask.
I'm very confident in the quality and manufacturing of the stain and the reputability of the company. If you ever have a question or problem, you can call them.
Hope that helps! I'm sure I'll be reading more from all of you soon! Thanks for being here!!! :)
-E

Here is a link that might be useful: Timber Ox Green - Home Page

We just set ours up-after coating it veeerrryyy liberally with linseed oil. Works great,smells great,looks great (and was actually a very zen thing to do;).

I've already volunteered to do the next one!!

I dont. I prefer them naked. Also gives it a rustic look after a year of weathering. They're holding up fine. dont have to worry about contamination.

I don't treat mine either. I painted the 1st couple I made before I found out about SFG. Then made many more with untreated pine which are now 8y/o and counting. So I don't see the point in bothering or the extra expense.

But I also wouldn't staple plastic to the wood either cuz that will trap in the moisture and likely rot faster. Flip them over so the contact with the soil is minimal. If you want you could flip both spring and fall.