The key to sealing a new redwood fence (after it ahas cured for a bit) is to use a premium primer and back roll it in to the wood while your spaying all the angles. Oil base primers used to be the only thing that would block the tannin from coming through the paint but today they have reformulated the some of the new latex primers to do the job. if you didn't bench prime it before you built it, Make sure you get to it before ivy and other foliage start attaching itself to it. Your going to want to dig a ditch at the bottom too as to make sure you protect the bottoms. Make sure your automatic sprinklers are off while undertaking this process. i recommend having the painting company tint the primer if your finish coats are medium to dark in color.