I have tried the baking soda and diluted milk methods on my gardenias, but
the sooty mold is still clinging to the leaves.

A Google search on the subject returned several suggestions about "white
oil." It seems that this will kill the insects that are creating the
problem to begin with, and further that it will serve to make removal of the
sooty mold much easier. Apparently, "white oil" is nothing more than a
combination of vegetable oil and dishwashing soap--although it can be
purchased commercially.

What has been your experience, if any, with this product, and would you
recommend it for my gardenias?

Many thanks!

Scarlett