If you've got a little patience and want to try something not that destructive, Coca Cola works really well (or any high acidic soft drink of your choice). Just brush it on and let it soak in to any gaps. Keep applying more, it'll take a couple of days. But this has worked for me and doesn't particularly damage the paint or the frame. And when I did it the seatpost was still useable afterwards, needed a lot of polishing though. I've also had quite good success with Citrus degreaser too, although try to keep this off the paint as its can be quite strong.
If you've got a little patience and want to try something not that destructive, Coca Cola works really well (or any high acidic soft drink of your choice). Just brush it on and let it soak in to any gaps. Keep applying more, it'll take a couple of days. But this has worked for me and doesn't particularly damage the paint or the frame. And when I did it the seatpost was still useable afterwards, needed a lot of polishing though. I've also had quite good success with Citrus degreaser too, although try to keep this off the paint as its can be quite strong.