Originally Posted by
oneclick
Note to all, as I see there are more than one -
NaOh is not an acid, it's a base.
Don't use acid - it'll attack steel.
Muriatic acid has long been used to dissolve smeared aluminum from iron cylinder walls after engine tuning failures.
Strong alkali solutions are known to embrittle steel, I've seen fractured cassette sprockets result from such soakings.
As was already mentioned, brute force methods can crack frame joinings.
I've used a lot of methods and have settled on use of a reciprocating saw with a suitably long/narrow blade. Always successful except with steel seatposts.
Don't cut in line with the clamping slot, and don't let the blade stroke exceed where the tip of the blade catches the opening of the tube!
Use a sharp blade and periodically probe the cut using a sharp, bent spoke to check for remaining aluminum which the sharp spoke will catch on.
The teeth will never even start to dig into the steel seat tube since there just isn't enough contact pressure to begin cutting alloy steel unless you're using a diamond blade.