Friends I have removed many seat posts that bike shops could not get out, I have tried many chemicals, saw blades, oxy/acetyl torch. I have witnessed frame builders and frame painters melt them out with torches.
If you get lucky and have very little corrosion you can put a bike upside down in a bench vice clamping on the seat post.
If you have lots of corrosion that extends 4 to 6 inches down the seat post, Chemistry will not eat down 4 inches in less than a year. I have only seen the erosion of a few millimeters of the oxidation with chemical remedies. This will work if you are lucky and have very little corrosion. When you are not lucky then a long saw blade used mindfully will cause the least amount of damage to the bike frame.
If you are going to repaint the bike then using an oxy/acetyl torch will melt the aluminum seat post out of STEEL BIKE FRAMES ONLY. The heat will destroy the paint and the torch is placed inside the seat post not outside on the frame seat tube. If too much heat is used the seat tube will crack or the seat stays will become unbrazed. Great care is needed and a bunch of skill if you are to use a torch.
Cold works best if the seat post is not inside the seat tube. When they are bound together tightly they are hard to cool at different rates. When they are cooled at the same rate they stay bound together. Once again if there is very little corrosion binding the two together, cooling may work.
The saw works the best for the least amount of damage to the frame. It is slow going and hard work, but it works every time.
Christopher Wallace

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