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.