dusting tomatoes

[tessa]

i have the bleeding mites again.
i've been spraying regularly with pyrethrum...but my two best plants are going down fast with the mites.
so i've decided to try tomato dust. it's a sulphur, copper, spinosad mixture. all of these things are, in my mind, organic substances....while not exactly being organic for gardening purposes.
i am, however, willing to venture outside my organic perameters rather than face loosing the bulk of yet another summer's crop of tomatoes.

but i have questions. when dusting tomato plants....
how thick should the dust be?
and how, exactly, do you dust the underside of the leaves?

i find that the dust simply sticks to the leaves...and doesn't seem to blow off. so i'm worried about it impeding photosynthesis, or become a cake.
am i supposed to rinse it off at some point?

dumb as it may sound....
i'm hoping for some helpful pointers.
there is much 'how to' out there about dusting principles.

thanks folks.