I filled the frame to the top with said "rust remover". Just for good measure I gave it a good shake and listened for any hissing. Thankfully nothing. That's the frame pressure tested then
I drained that, and re-filled with neat petrol. Gave this a good swilling, and repeated.
I mixed a very strong solution of petrol/Castrol R40 and continued to fill and drain.
Each time pouring the mix through my "Mr Funnel" to clear any debris. This whole process went on every evening for a week. Usually until the fumes became too much !
The first few fill and shake rotations went from this .....
To a clear mix.
Not a job I would wish to repeat in a hurry !