Re-using sprayers after using weed killer

I used resolve on a buddliah root that was forcing its way up between my fence and the footpath outside, it wilted after a day, but continued to look alive, but now a few weeks later all the foliage has browned and withered, this was a stump maybe 6" across that had re-sprouted after it was cut to ground level and started to hump up the tarmac on the pavement, hopefully no longer a problem.
Grendel