Seatpost stuck - where would I get Caustic soda from?

the answer to anything stuck to anything is heat. just pour a load of oxy-propane heat on it and pull. it will move. of course the paint will be utterly buggered, but the surfaces of the metal will not.

Well, except in the unlikely event that it's aluminium inside steel, because aluminium expands more than steel, so heat would only make the problem worse. I can't think of anywhere that might happen. Maybe some kind of aluminium pole, or post, inside some kind of steel tube...? No, I agree, seems pretty unlikely.