I'm an industrial and commercial painting contractor. The problem you will have with coating the pipe is it's made from galvanized metal. There are VERY FEW products that will stick to it. Most that will stick are not suitable for wet applications. If it was mine, I would go with the plastic pipe insert. You can get a product called STAMPEDE at Sherwin Williams that comes in a caulk tube to attach/seal the plastic to the metal.
