Do NOT attempt to do this yourself unless you are trained in the use of the proper tools and have them. I have seen way too many perfectly good frames ruined by DIY'ers. If you take it to a shop and they pull out an adjustable reamer take it to another shop. The chances that they might actually know how to set up an adjustable reamer at a bike shop is infinitesimal.
If you want to clean the inside of the tube use a non-abrasive like scotchbrite and figure out some way to clean out the trash you make before you clean it. Certainly before you stuff in a seatpost.
I had a BobJackson that some moron had originally assembled and it was a major effort to undo the mindless damage he had done. Steel is not impossible to repair but it ain't easy and it ain't cheap.
An inside micrometer will tell you what the diameter is way better than cramming in a buncha different seatposts.
Charlie