It was all a bit Breaking Bad…
Here’s a pic of the set up, I messed around a bit to get a system I was happy with. I was desperate not to damage the paint, it’s already a bit iffy but good enough to keep.
I used caustic at around 35/40g to 200ml water.
Bunged up the end of the seat post with blu-tac or plasticine. Poured the caustic into the seat tube via a funnel piped to the bottle cage mount. Once the tube was full to the bottle cage mount then allowed the pipe/funnel to rest (lower than the fluid height) over a Pyrex jug with water in it. I then bunged up the seat tube hole in the bottom bracket with a bit more blu-tac, this meant the gas created was vented out of the funnel and no bubbling juices came anywhere near the paint if escaping from the bottom bracket.
There was a fair bit of bubbling/frothing/steaming/spitting and everything got rather warm as the reaction progressed.
The mucky liquid which bubbled out of the funnel was cooled and diluted immediately as it dripped into the water in the Pyrex jug.
The frame was suspended over a seed tray with water in it, to catch and dilute anything that I might spill or drip.
The reaction time was about 3 or 4 hours, once everything has college and stopped bubbling I removed the plug from the top of the seatpost and the inside of the bottom bracket. Drained the remaining fluid /sludge (if anything was left) before repeating.
It took 3 days, probably dosing it 2 or 3 times per day.
I did initially try a stronger solution, but the reaction was too powerful, all the solution bubbled furiously out of the pipe within an hour, quite violently. The weaker solution seemed to give more reaction time with the solution in contact with the aluminium to do the job.
I kept plenty of water on hand to make sure any spills of accidental contact could be washed immediately, off both me and the frame.
Very impressive result.
Patience and preparation are key.
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