Borate-Based Wood Preservatives
Bora-Care is a borate chemical in water that is relatively safe but cannot be used in locations where the treated wood is exposed to water as the borate will leach out as easily as it went in. So, it is my guess that this will not work for your indicated use as in most cases rain will hit the porch. This chemical is applied wet and then the wood must be kept soaking wet to allow the chemical to diffuse into the wood. Several applications may be required to achieve thorough penetration and adequate levels of the chemical in the wood.
From contributor S:
Borates will diffuse through seasonal moisture movement, so it is not entirely necessary to keep the surfaces wet after treatment. The main problem is getting the loading in the first place. You would need to immerse the items for 60-70 seconds in the borate solution to get adequate initial uptake.
The other way is to use a concentrated form, such as a paste or borosilicate rod. These are inserted into drilled holes that are plugged or filled afterwards. As the moisture content rises towards the point where decay can occur, the borate dissolves and disperses throughout the danger zone.
Borates will leach out if there is a suitable "sink" to supply a higher moisture content zone (standing water or wet soil). There will be leaching from the surface by the effects of rain, but recent studies on borate-treated timber in abandoned building projects in the USA and Australia have shown that a critical level is reached where leaching almost stops. Coating the timber stops any leaching.
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