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Sounds like a plan, have you bought your caustic soda yet, I got mine from Robert Dyas, was only a couple of quid. I reckon mixing the soda crystals with hot water first to make a solution and then pouring it in is the way forward, imagine it'd be better than trying to pour them directly into the seat tube. This way you can also experiment with different concentrations until you find one which gives a controlled reaction. For bungs old wine/champagne corks should do the trick, the hot solution would soften/melt blu tack etc.
GreatSince78-
I'm looking at plastic pipe over the seattube end and I need to make some sort of seal around that and the top of the seattube, fingers crossed only the paint on the very top clamped bit is damaged which could then be covered with the seat clamp.
The diy crownrace plastic waste pipe tool I'll be cutting short is like this.
I just need to work out a suitable material to go between that and the end of the seattube that I can jubile clamp around, unsure about using innertubes tho lots of people seem to be using them to protect from chemicals.
Solution will be 2:1 water:crystals, has been suggested by a few people as a good mix.
Clockwise
This seatpost is a fair bit thicker. I think I'm going to make a setup with the pipes I have about so some waste pipe that was suitable diameter for crown race setting and I'll get an end for it. With any luck I'll be able to make a bung/cup/end to go over the seatpost and pour in from the BB side with a funnel.