I am building some garden boxes out of Cedar. I'd like to protect them so that the wood lasts longer, but I am concerned about certain finishes that might leach contaminants into the soil. Is there a clear winner in this regard? Are there finishes specifically designed with this in mind?
There are two substances that instantly come to mind:
- Wax (paraffin or beeswax)
- Mineral Oil
I use both of these to protect my wood butcher block. They are both food safe, non-reactive and even relatively inexpensive. They will alter the appearance of the wood, by making it more lustrous.
I have even seen some people finish their butcher blocks with hot paraffin, using either an iron or some other gentle heat source. I cannot speak to the longevity, but by sealing out moisture and barring against some amount of microbes it would certainly improve the lifespan, although by how much I cannot say.
For garden boxes, most of the rotting will come from the inside, not the outside. As such, the best protecting substance is probably plastic. You will not be able to reapply waxes or oils on the inside (unless you're planning to remove the dirt regularly, which you could do); a plastic liner will last longer than those by years. Of course, you need a food-safe plastic, and even this won't perfectly prevent rot - you'll have to be careful to allow it to drain properly.
This question on Gardening stack exchange explains this in some detail.
"Food-safe" finishes are generally those used on surfaces that will be in direct contact with food, such as butcher blocks, salad bowls, etc. For outdoor use, any good tough outdoor finish should be fine. As long as you put on thin coats and allow them to fully cure before putting on the next coat, you shouldn't have to worry about anything leaching into the soil. Really, the leaching danger mainly arises from using pressure-treated wood, especially if it's still wet when you place it.
Most typical wood finishes are safe ONCE CURED. Here is a somewhat relevant article by Bob Flexner http://www.woodcentral.com/articles/finishing/articles_497a.shtml.
Once cured a drying type finish, varnish or paint, will not leach into the ground. Since this will be exposed to direct sunlight use an outdoor product, either paint or opaque stain. The only way to protect wood outdoors from UV exposure is to cover it with a pigment. The more clear "UV protector" finishes will have to be recoated every 2-3 years. Pigmented finishes will last 2-3 times as long.