What to pour down it

[Shut Up Legs]

Thomson, who make excellent seat posts, suggest that you remove and clean a post every 90 days. While this will not help you now it may be something for the future.
this you tube entry may help
View: http://www.youtube.com/watch?v=54WhdrLKKvY

I'll have to try that successive heat and cold method myself, as I also have a stuck seat post. Mine's on a touring bike, though, with CrMo alloy frame and alloy seat post.

 

[raleighnut]

I'll have to try that successive heat and cold method myself, as I also have a stuck seat post. Mine's on a touring bike, though, with CrMo alloy frame and alloy seat post.

CrMo alloy is a steel blend Chrome/Molydenum Steel to give its full name. Mind you the seat post is probably ally (Aluminium) !

 

[HLaB]

I'll have to try that successive heat and cold method myself, as I also have a stuck seat post. Mine's on a touring bike, though, with CrMo alloy frame and alloy seat post.

Check Sheldons link in my post above but I think its better to do the opposite for a steel frame.

 

[Gravity Aided]

I have had some luck in the past with hot vinegar as well, may be a try before caustic solutions.

 

[sidevalve]

Alloy post - use a very coarse hacksaw vertically into the tube [be prepared to lose some blood and be carefull of the frame] then crush to allow the WD40 etc to get in. Failing that upend the frame, bung up the end and fill with diesel - then LEAVE it for a week or so. It's a bit smelly but may be safer than caustic, anyhow it's cheap enough and you can always try the caustic later.

 

[Smurfy]

I'd go for Caustic Soda straight away. It's a proven technique, and if the seatpost is well and truly seized the other methods are unlikely to work for the reasons stated by Uncle Phil (conversion of aluminium to aluminium oxide involves a volume increase), which means when you've tried everything else and found it doesn't work, you've already got a load of other crap in there when you tip the NaOH in!