I'm putting up a 8x6 Winchester apex shed near to a tall wall in my garden. I understand that I need to treat the wood regularly. The shed was very cheap and the wood is not the strongest. I'd like to get the shed as close to the tall garden wall as I can. Is there something I can use to coat the wood which will be closest to the stone wall so that I am able to push the shed right up close to the stone? If it can last 20 years, that would be good enough for me.