I flux the fitting only, using the end of the solder to apply the flux. I know you should apply to pipe, but if its already clipped to the wall its a right carry on, plus I always flush after and never had a problem. So middle finger up to the rule book.... Last time I used a brush I went through a phase of getting leaks which i'm convinced was dirt on the brush getting into the fitting. As soon as I binned the brush and went back to my old ways, no leaks!
Also If I think an old bit of pipe is going to give me grief, I tin the end first then wipe, then make up joint and crack on, always works.