I've got a small pile of reclaimed 2x4's that were originally
purchased to build shelves in my basement. There is a thrift store
locally that gets the wood out of homes that are being demolished. My
parents ended up getting us heavy duty plastic shelves as a housewarming
gift, so now the lumber sits there unused, taking up space.

I also have a need for some bed edging in my yard - to keep me from
mowing the mulch around a couple of fruit trees and a blueberry hedge.
I'm interested in using some of this ~ 80 year old wood for the job.

Can anyone recommend a good wood preservative that I can apply with a
paintbrush that would stand a good chance of helping the wood stand up
to constant contact with the ground and moisture for perhaps 6 years?

Thanks!