Preserving Timber Posts
Here in the sunny south summers are either scorching sunlight or thunderstorms. My customer has used old hand-hewn timbers for the porch posts, thus the old wood is weather-split, twisted, somewhat cracked,and generally what you'd expect from 100 year old timbers that were once interior parts. The wood is solid and sound and strong enough. The problem is that they now need weather protection yet we don't want to see any changes in color or finish from the natural state. The finish will need to penetrate the cracks and thus will probably be sprayed heavily to be sure it gets in to all the voids.
Thompsons would probably work but will likely darken the wood and it has to be renewed every couple years around here. Being that we don't want to see any finish and it needs to penetrate the wood common finishes like Poly and varnish are out too as they will show some kind of sheen. These are already installed so vat-finishing is also out. Anyone have experience with doing something like this?
Re: Preserving Timber Posts
I did not see this post when you first posted it. The product that I would recommend is Timbor. It is a sodium borate solution. It comes in a powder form and you mix it with water. You can spray it on with a garden sprayer or brush it on.
There is another product called Boracare that was used on several TOH projects, it is basically Timbor mixed with ethylene glycol (antifreeze). The only issue with this is that it has a tendency to restore the original finish of the wood, that is it makes it look newer, almost like fresh cut.
Both of these products are water soluble so they have to be reapplied once or twice a year, unless the posts do not get rained on. The sodium borate solutions not only kill and prevent the return of molds, it also repels insects such as termites and ants.
There are borate rods that can be inserted into the posts that will dissolve into the wood every time it gets wet that can provide long term protection, but that may cause a visual issue that you seem to want to avoid.