The following worked for me. Al seatpost in steel frame. Pretty seized. Local bike shop tried and couldn't budge it.
Some of what I will describe goes against the advice here (Heating Aluminum), but it worked for me.
- Got a handheld propane torch (~$25 from your local hardware store), a temperature gun to measure, and a two cans of compressed air from local dollar store.
- Removed the seat.
- Heated the seatpost up to about 500 deg (F), and a little bit of the top of the steel seat tube. Didn't really damage my paint much.
- Then, turning the can of compressed air upside down, used it to freeze the seatpost, and top of seat tube.
- I then whacked the top of the seatpost quite hard with a hammer, to drive it further into the frame, about half an inch, to break any "seal" of Al2O3. Add cutting oil (WD40) or ammonia while hammering the seatpost - this way the oil actually gets into your seattube...
- Repeat steps 2 & 4, if needed.
- It was a little loose now, for the first time I could twist the seatpost.
- To get better leverage, and avoid breaking my seat - I just wrapped - very very tightly - a long hank of 850 Paracord around the seat post - yes this will grip aliminum, if you clean the seatpost first with alcohol. (Can sand your seatpost if you are desperate)
- I then just tied a simple slipknot in the end of the paracord, and stuck a breaker bar in it, perpendicular to the seatpost, and that gave me plenty of leverage. (If you dont have a breaker bar, a simple metal-body hammer, or something else strong will do)
- I was then able to turn the seatpost full revolutions, breaking the seal, then work the seatpost out using a bit of elbow grease...
TL;DR: Use a cheap form of heat and cooling to expand and contract the metals, and a bit of impact, to break the seal.