Everything I have read says to apply weed killer after washing the block paving and before sanding but all weed killers I know say to apply to the leaves and it will then kill the roots. Surely it would be better to spray the weed killer then wait a week and then wash rather than wash then weed kill? I take it that it is the time factor that stops people sprays a week in advance? I am thinking about not using weed killer if I can't spray at least a week in advance. Am I missing something here?