Where did you get that "Bartoline" caustic soda? A hardware shop? It looks like proper strong stuff unlike the weak soda I got on the high street!
Great for cleaning chains I've found.
I got it in my local general shop, 'Super Value' in Cottingham, for £1.50 per 500g container. I bought 3 so that I would have plenty, but one was enough to do the job. I did put a little bit from the second container down, just to check if there was any aluminium residue left, but there was no reaction at all.
Great video. How smooth is the seattube now? Have you tried putting a seatpost in?
I tried the correct size seat post this morning and it goes in fine. The inside is smooth, with no trace of aluminium left, though I'll probably give it a final clean before I use it.
I think the Bartoline stuff is from B&Q.
The hotter the water is when you put in the caustic soda the strength raises exponentially. It is very dangerous though and produces a very violent reaction. you need a face shield, goggles, gloves and non-fabric apron etc and it should be poured in from a distance. If put into water when it's near boiling it will explode.
I would not recommend anyone mixes the caustic soda with warm/hot water. The hydration of sodium hydroxide when added to water is highly exothermic and pretty rapid. The squash bottle I was using got melted out of shape by the heat and I'd probably suggest stirring it in, rather than shaking it to mix it, as I did in the video.
Also, as a note, if you are intending to try it, make sure you are adding the caustic soda to the water, not the other way around, as that would also react dangerously quickly.