Alcohol and salt
I shake it shake it shake it the salt acts like a brush because it doesn't dissolve in the alcohol.
Discussion
I need better muscles for the shake lol my corners never clean up like yours
Run hot hot water on it as you shaking it. I shake a little run a little hot water on the outside of the bong then i shake it then run a little hot water over it shake it. Hahaha finger tips 🤣 get it but nothing like clean glass. 😅
I'll have to give the hot water a go or might just try torching it for a second to get some heat there. That's a good idea on the added heat to loosen things up.