Like minty said, follow the instructions on the packet, no need for any other treatment.
Ummmm "I'll find out the name of the weed killer my OH uses at work. I used it quite casually....3 days later everything was dead."
The problem I found with some of these very fast acting weed killers, is it looks very dramatic to see the weeds keel over so quickly, however, I think they only seem to kill the leaves off and doesn't seem to kill the roots, as a a couple weeks later they are all back again.