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 & 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 & 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!
bicycles.stackexchange.com/users/23/Dustin Ingram
This is a tough one.
It's not impossible
but you really need mechanical advantage.
If you can clamp the seatpost, you will get the best chance at freeing this.
You will probably damage/lose the seatpost with this method.
Make sure the seatpost is free ofgrease and oil on the outside.
Turn the frame upside down and clampthe seatpost into a bench vise.
Spray
WD-40 into the seattube fromthe bottom bracket opening.
It will probably be dripping rusty coloredoil out the top of the seat tube (which is now facing the floor - you might want to put something to catch the oil under there.).
Carefully hold the head tube and therear stays and try to rotate theframe on the post.
Work
it slowly and carefully beingsure not to bend the stays!
* Make sure the seat post is notrotating in the vise (if so, clamptighter)
This process can take some time.
You will need patience and lots of WD-40.
Eventually, the post will rotate in the seat tube and you will be able to work it out.
*
The trick is knowing how much force to apply.
Sometimes you need more than you think and sometimes you apply more than you need (which results in unwanted damage).
This difference comes with experience.
Oh
and also: grease the new seatpost well before you put it back in
- you'll appreciate it later.
bicycles.stackexchange.com/users/87/42-17
You should check out the 15 Way To Unstick a Seatpost by Sheldon "Unggggghhh!"
Brown http://www.sheldonbrown.com/stuck-seatposts.html
bicycles.stackexchange.com/users/1493/Aleksandr Levchuk
Untested and at-your-own-risk, but this is what I would try: Soak the rusted join with WD-40 from above and below (ie.
flip the frame and spray down the seat tube).
Let it sit for a few hours or overnight.
Secure the frame against something solid, like a heavy workbench (not a repair stand!).
Put an old saddle you don't care much about on the seat post.
Hit the side of the saddle's nose with a hammer or mallet.
Use short, hard taps -- using wild swings you're liable to hurt yourself or the frame.
The idea is to use the torque of the saddle to break the bond between the post and the frame.
You don't need to move the post much.
If the post moves, then you should be able to work it loose without the hammer.
If it doesn't, then worst case you've destroyed an old saddle.
bicycles.stackexchange.com/users/157/darkcanuck
Cut the top off of the tube then carefully use a hacksaw blade inside the seat post to cut toward the seat tube.
A handle for a hacksaw blade works or just wrap the plain blade with a bit of tape to make a handle.
You should then be able to free the old seat post.
Or does the seat post extend way down into the frame?
Even then this should help as the WD-40 will get a better chance to reach the lower parts.
bicycles.stackexchange.com/users/112/John Dyer
At the non-profit workshop, where I volunteer, we sometimes ream out the old Seatpost enough to stick in another, smaller-diameter Seatpost.
Especially if you don't care too much about aestetics and the seized seatpost reaches deep into the frame(at least 8-10cm, preferably more) this might be a viable option to just make the bike rideable again.
If it is less, you might be able to saw a slice out of the old seatpost(would have to be all the way into the downtube of the frame) and the crush the compromised tube with vise-grips.
In my experience, seized seatpost and their removability often boil down to how much effort and time one is willing to invest.
bicycles.stackexchange.com/users/39007/MTTI
I had this problem with a very stubborn seat post (turns out there was a wedge in it to keep the post in).
One thing that I tried was putting a blow torch on the frame to try and expand it a bit to get the post out and loosen any corrosion.
I would only try this after WD40 has failed.
Disclaimer: this may damage your bike
bicycles.stackexchange.com/users/134/sixtyfootersdude
there is aerosol spray available at most hardware stores called PB Blaster
it is much more effective when dealing with rust than WD=40
bicycles.stackexchange.com/users/2192/mikes
Try put your seatpost / saddle on the vise ( reverse the bike ) then rotate the frame for pulling out the seatpost, put WD-40 or other lubricant for before you do it....
bicycles.stackexchange.com/users/3189/Rick Ant
I have an old steel frame (Columbus Number 2 I think) that has the seat post (Suntour Superbe Pro - see, I said it was old!) rusted in. I have tried lots of WD-40 but it will not budge. Any tips on how I can get it out without having to cut the frame up?
Removing Seat Post rusted into downtube
bicycles.stackexchange.com
<frames><seatpost><rust>
bicycles.stackexchange.com/questions/418
bicycles.stackexchange.com/users/106/Anthony K
To remove a rusted seat post use WD-40 or ammonia and leave to sit. Then clamp the seat post and rotate the fame to pull out the rusted seat post. It would be a good idea to start with applying a lubricant, such as WD-40, or ammonia if that fails. To remove the seat, you could try turning the bike upside down, clamping the post and rotating it. Moreover, consider using a blade within the seat post to free the way towards the tube. It is also possible to insert a smaller seat post in the old one. Finally, you could try using a blow torch to expand the frame and subsequently remove the post
It would be a good idea to start with applying a lubricant, such as WD-40, or ammonia if that fails. To remove the seat, you could try turning the bike upside down, clamping the post and rotating it. Consider using a blade within the seat post to free the way towards the tube. It is possible to insert a smaller seat post in the old one. You could try using a blow torch to expand the frame and subsequently remove the post.