Ian,
a number of years ago my job was crop spraying ,
basicly it was the spraying of crops for any number of reasons, ie. pest control, fungicidle control and weed control amongst others, alot of this was done by air using helicopters, now the only solution for controling weeds by air was infact cccp mixed with deisel , " and what a good job that did " nothing else was used because of safty issues. this mixture was used to control bracken in scotland, yes a weed! " a weed , any unwanted growth " not ideal for spraying on paving etc either
but , the bes t by far for controling weed was round up , a systemic killer, meaning it takes time and transfers through the weed and into the root, kiiling it capability of regrowth , unlike a number of others, gramoxone etc, these just got rid of the visible growth on contact , but the conditions for any of these to work needed to be dry as the systemic took up the potion into its system, " a bit like the plant having a good drink" usualy a couple of weeks ideal for full effect.
So the conditions need to be dry, but not arid without rain been imminent for 24 hrs .
cant understand why your saying the weeds need to be wet, not disbieliving you , i havnt used soduim clorate to be honest, this is a granule is it not ?
geoff