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Mine is aluminium on aluminium so I assume it's worse
Definitely the worst of all possible worlds. Don't even think about chemistry, you'll either have no effect or you'll destroy the frame first as it has the thinner walls.
The seat post is toast if you want to save the frame. Depending on how far gone it is, the frame might be toast too. What you need to do is machine away the seat post from the inside out. The "big machine" method is a horizontal boring mill. Since the seat post is round and straight, it would be a fairly easy set up to get the frame on the table with the seat post coaxial with the spindle, the main caveat being to remind the machinist just how thin the wall of the tube is before he puts too much gronk on the workholding solution. Once you have it held in the right place relative to the tool, it's just a matter of taking light cuts with a boring bar until the hole is the same diameter as the seat post was. If you're left with a shiny bare metal bore, you've won. If there's still a fair bit of powdery crap in the bore before you break through to actual seat tube metal, you've just wasted your time 🙂
gbj_tester-
Yeah hold on, let me go the garage and start the machine right up!!
Looking at the condition of the rest of the frame i recon its in for good. Im going to try some chemicals first as this frame owes me nothing and its worth a shot.
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Okay so the seat post is free, a little gt85 and twisting it in a vice free'd it up.
However it only twists and wont come out or in. No matter how much I pull on it which is a PITA
Josh
Mine is aluminium on aluminium so I assume it's worse off than carbon/alu etc.
Im going to buy some drain cleaner tomorrow to see if that works, if not its caustic soda. (Which may kill my frame too though)