Just buil a frame and dunked it for two hours to try soak the flux off, water turned a funny colour and the flux seemed to be gone and nothing seemed to be left after a wipe with my thumb. Would it be safe to assume that its clean inside the tubes?

Did you put fairy liquid with the water… It really helps dissolve flux in those hard to reach places.

I’d definitely be thinking about scrubbing the tubes with a brush and water/fairy liquid, then rodding them out using clean rags with clutch + brake cleaner or similar after all flux has been removed.

I did not use fairy liquid, saw nothing mentioned as such on other forums when searching for it. Id struggle rodding some of these tubes! External brushing is easy just the ineternals will be a pita.

I’m not sure if having flux on the inside of the tubes would have a negative effect on steel tubes anyway, it is providing an atmosphere to prevent oxidants.

It may have an adverse effect on any frame saver- however I am no expert on this!

The issue with the flux is that it is an acid based flux, so unless neutralised or removed can cause corrosion itself, removing the rust is pretty useless on the internal tubes and the frame saver prevents any extra internal rust past the surface rust.