Why deck finishes fail before fence finishes do
Water is the main reason. A vertical surface sheds rain in seconds; a deck board holds it in the grain and around every fastener until it evaporates. Add direct overhead sun, which breaks down the binders in a finish faster than angled light, and foot traffic that physically abrades the surface, and a deck is being attacked three ways at once. This is why we put an additional coat on horizontal surfaces rather than treating the whole structure identically.
