Rust on edges will result in rust creeping around the edge to under base. Oil is good but a film of petroleum jelly will provide a better film ,wax the base well then apply jelly to bottom and edge of metal. If the board is dry when applied you will never see rust even if your water pipes are dripping from condensation this summer.Same can be said for any minor nicks on metal boards, oxydation can start on exposed bruises along the side too:smashfrea so apply petrolium jelly to those as well.