I've just built a dwarf wall out of stone for our new lean-to greenhouse and will be fixing sills made from 200mm wide 47mm thick treated kiln dried POR, with one long length 4.2m and two short lengths 2.4m and 1m. They will be chamfered and I intend to mitre them then paint with Cuprinol Garden Shades. My question is how best to join the mitres, will external PVA be good enough?