The Handyman's Pocket Book: Rustic Work.

There is no way of preventing bark from stripping.  The best way is to give a coat of size then finish with copal varnish.  Apply the varnish before the size completely dries out as it then mixes with it.  Best rustic work is coated with linseed oil first.  Larch is not so liable to strip its bark as other trees.  To finish peeled rustic work go over with concentrated size coloured (if required) with brown umber.  Follow with a good outdoor quality oak varnish.  Ash or chestnut tend to split and should be varnished as soon as possible.  Larch should be left with the bark on.