Lots of threads of frustrating seatpost, but one problem not often mentioned is scored aluminum post caused by a gouged up tube pinch clamp. Also deformed or ovalized tube can make it difficult. Sometimes no amount of penetrant helps and when really messed up, safer to just cut it off, then cut slit in pieces. Good luck!

Congrats on being able to twist it! Part of the problem is all the particulates in the tube. They do act like sandpaper and prevent extraction. I don't have near the experience as say wrk101 but the post I removed was stuck badly and was a challenge to remove after getting it to rotate. My approach was to place my stocking feet on the DT and chain stays while pulling and rotating the post with a saddle, I didn't care about. It was very difficult and became easier as more of the post was exposed.

Once it moves out, check for gouges as mentioned by [MENTION=350383]crank_addict[/MENTION] to determine if you are getting lug interference. If that is the case, rotate the post with a flute located by the offending part of the lug and either try to extract it with back and forth rotations or GENTLY pry the edge away from the post enough to remove the interference. Usually it is a part of the lug that looks like a burr and can be eliminated if carefully done, no BFH should be applied!

Continue to add lubricant in the flutes to flush the junk out of the tube to reduce the resistance from the loose particles.

This is a trek 510/20? can't see the previous page. If so, the tube is straight gauge so you shouldn't have problems with the lower portion of the post corroded larger than the lug area. An option is to drive it in a 1/4" and then see if it will pull out easier. Just a dumb suggestion that might work!

Part of my solution was to apply heat to the post head. Not so much to melt the metal or burn the paint! Expansion with contraction can do wonders too. Use the Freeze Off to reduce the diameter of the post. I would not be in a hurray to pull out the hacksaw blade. You may want to let the PB Blaster soak during the move. My post soaked for weeks before I applied Kroil, letting it sit for a couple of days (week?) and then heat with full resignation that I would have to pull the blade out if I didn't get some kind of movement!

AHH found a pic after pulling the post!
Before:
[IMG]P1000939 by superissimo_83, on Flickr[/IMG]

After:

[IMG]P1010071 by superissimo_83, on Flickr[/IMG]

Nothing breaks corrosion like equal parts ATF and Acetone........Nothing

+1 on the transmission fluid and acetone. Best penetrating oil I've ever tried. I'm reading this thread and wondering if this condition is exclusive to aluminum seat posts. I've never had this much trouble with chrome plated steel.

Pull the bottom bracket and cranks off the bike...then dump your magical solvent in there...just spraying is not enough. Then it ought to release.

All the stuck posts I have ever had were AL

A lot of times when you have dissimilar metals, al seatpost in steel frame, you get galvanic corrosion where the seatpost essentially bonds to the steel frame through aluminum oxide and that is what you have to release. That's why ammonia works since ammonia eats the aluminum oxide.

Still working it back and forth with a cheater bar and it seems to be getting a bit easier. Keep working it and spraying penetrating oil into it but still can't get it to come out any. Don't have a big bench vise to use or I would give that a shot.

A large bench vise is so very useful in a workshop. Head to the co-op, or better yet, get a large vise and bench. This will not be the last time you need it. If it moves (twists), it will come out. I did have one (out of about 50) that twisted but did not come out. I ended up having to drive it in about an inch (risky move). Apparently it was hung up on a chunk of corrosion. It came out immediately.

I cannot imagine attempting this without a vise. You need a vise holding the post still while you twist and pull upward simultaneously.

VICTORY!
Lots of PB Blaster, Marvel Mystery Oil and leverage. Only casualty was the stock Avocet Racing II (I could probably repair if I tried) and a really marred up seat clamp area on the post.

And of course now the seatpost can go all the way into the frame up to the clamp and comes back out without a hitch...

Get a small brake hone to clean up inside of seat tube.