How to Weather Lumber Using Ferrous Sulfate (Aging a wooden fence)
When you put up a new fence, the new wood really stands out. Instead of waiting for Mother Nature to grey the boards, just use inexpensive ferrous sulfate. It works on any wood. The ferrous sulfate reacts with the tannin in wood and grays it. For woods like pine that have little tannin, you simply add the tannin manually by spraying it on.
You can see from the photos below how a ferrous sulfate solution worked at making fences gray. I've used it on cedar, oak, and pine board fences as shown.
Oak fence after and before ferrous sulfate treatment
Cedar post before and after ferrous sulfate treatment
Pine boards on a fence, one side treated with only with ferrous sulfate the other side with tannin follwed with ferrous sulfate.
Direction on Aging Oak Boards
Purchase a 1 lb package Ferrous Sulfate 
from Amazon (It is also sold in garden centers). The ferrous sulfate reacts with the tannin in wood and grays it. Mix 1.4 oz (2 tbl.)* of the ferrous sulfate in 1 gal of water. Then simply wet the wood you want to turn gray. In a few hours, your fence will be an attractive gray. You can also preset the fence and then apply the solution. People have also dissolved steel wool in vinegar to produce ferrous acetate which also will age wood in the same way ferrous sulfate does.
* I used 2oz/gal on an oak board fence and it turned the wood dry dark. You may want to test the concentration before you start your project.
Direction on Aging on Pine Boards
Purchase a 1 lb package Ferrous Sulfate from Amazon (It is also sold in garden centers) and one package of tannin. Since pine has very little natural tannin unlike oak, you have to manually add tannin to the wood's surface. Mix three tablespoons of the tannin in one gallon of water and spray it on the pine boards. After the pine is dry, mix 2 oz (3 tbl.) of the ferrous sulfate in 1 gal of water. Then simply wet the boards. The ferrous sulfate reacts with the tannin to produce an an attractive gray.
* I used 2oz/gal on an oak board fence and it turned the wood dry dark. You may want to test the concentration before you start your project. Woods like oak that have a lot of tannin need a less concentrated solution compared to pine.
Waterproofing fences with Wohlman's Classic Wood Preservative
After you have grayed your fence, you can add rot and waterproofing protection by using Rust-Oleum's Woodlife Classic wood preservative. This clear formula repels water and contains nearly double the fungicide of standard sealers or stains. It can be used as a final finish or paint primer. This is the same sealer/preservative we used as a wood conditioner before painting. Of course all wood will breakdown in sunlight.
