If it's an alloy post in a steel frame then you can also try plugging all the holes in the frame and filling the seat tube with bicarbonate of soda, this eats away at the alloy but the steel is unaffected.
@Booga started
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If it's an alloy post in a steel frame then you can also try plugging all the holes in the frame and filling the seat tube with bicarbonate of soda, this eats away at the alloy but the steel is unaffected.