I found a very minor ding in the seattube of a Nishiki I just picked up. It's hard to notice, but annoys me nonetheless. This is a very short frame but I do not believe the current seatpost reaches the ding. I'm assuming that I can just get a longer seatpost in the correct size and essentially pushout the ding in the seattube with the end of the seatpost as I push it in. Is this correct or will this not work, or worse, will I screw something up? Thanks!
What may stop you from pushing out the dent is if there are water bottle bosses above it. Also, an aluminum seatpost is not going to be strong enough to push out a dent in a steel seattube. There are specific mandrels available used to remove dents from shotgun barrels but they are hardened steel. Frankly, if it's small almost to the point of being unnoticeable, I'd leave it alone.
If it's really small, then just bondo it and put some touchup paint over it. Tiny dings in a seattube aren't anything to worry about. But then again, if a little ding is upsetting, then a spot of touchup paint will be no better. Personally, I'd just leave it alone.
I was bothered by a top tube ding on a bike that I bought recently. I used blocks to roll it out, but there is still a little bit of a ding left. I can say that the ding gets much less noticeable as time goes by.