Broken and corroded/stuck seatpost

Block the bottom bracket openings and base of the head tube.

Block the bottle mounts and any other holes- bluetac should be fine.

Pour concentrated caustic soda into the frame until full.

Leave the frame in a warm and humid place for four days- the seatpost should be gone.

Only do this if you have an aluminium seatpost in a steel frame, as it is the aluminium that gets eaten.