Probably most novice framebuilders use flux and getting it off can be particularly tedious. I admit that I have only used cycle design fluxes for some time, specifically because I find them a lot easier to remove. The only effective method of removal seems to be soaking the fluxed part in water until it loosens sufficiently to be brushed off. Hardened flux that has not been soaked off seems to set like concrete and otherwise requires a lot of heavy filing to remove. The hotter the soaking water the quicker the flux comes off but I have found it takes at least half an hour of initially boiling water to get brass flux to a brush able state. Wire brushing is most effective but can scratch thin stainless steel tubes unacceptably.