should it be treated during flowering???

i am about to water with epsom salt due to a magnesiem problem(I hope).I read that foliar spraying them works faster but seeing that i am entering week 3 of a 65 day strand is it ok to foliar feed or never while in flowering...

thanks for any help or tips

and can i dissolve it in regular cold water?does it have to sit for a certain amount of time??also do i put any nutes or just water and the epsom salt???