I have in the past on a few restoration had to sand down by hand using wet & dry sandpaper if it's just reducing one size, start with a course paper & then work up to a fine paper, this will reduce the diameter of the seat post! when finished it will look as it does now but smaller! remember to keep checking the size so you dont go to small always remember you will have to polish with fine paper so dont go to far with the course paper :-)
I have in the past on a few restoration had to sand down by hand using wet & dry sandpaper if it's just reducing one size, start with a course paper & then work up to a fine paper, this will reduce the diameter of the seat post! when finished it will look as it does now but smaller! remember to keep checking the size so you dont go to small always remember you will have to polish with fine paper so dont go to far with the course paper :-)