We have an old house... 1900's. There is a vent pipe running up through the bathroom to the upper apartment. When I took off the plywood cover to repaint the amount of corrosion is disgusting. Can this just be sanded down and repainted or do I need to call a plumber and have this vent pipe replaced? Is it possible to replace them? The other vent pipe in the front part of the house runs through a closet and has no such corrosion.

This is a cast iron pipe that the tar coating the pipe has gone down to nothing. Sand and repaint the pipe and you should be fine. Good luck, Tom