Removing Seat Post rusted into downtube
Before you take any more forceful measures, it may be a good idea to think a little about chemistry:


'Rust' is typically the name put on any type of corrosion, but technically &amp; specifically, it is the corrosion of iron (or steel) to form iron-oxides.
Although your bicycle is most definitely steel, your seatpost is not -- it's aluminum-alloy, which does not 'rust', but definitely corrodes (creating aluminum oxide).
WD-40 is a penetrating oil which is designed to, amongst other things, break up iron-oxides. However, it is pretty much useless against aluminum-oxides, especially when they have already corroded &amp; bonded.
Ammonia, however, is spectacularly good at eliminating aluminum-oxides.


Therefore, if WD-40 isn't doing the trick, get some ammonia, apply it in a similar fashion, and let it sit. It may just be all you need!