For a single picture, spray would probably best be done in postwork.
There are ways to handle drops in Poser, but the sheer number of
particles would be a lot more work than a good Photoshop filter.

Alternatively, you could make a shape corresponding to the outermost
boundaries of the spray, and give it a transparency map (transmap)
with lots of dots.  This would be more work than the filter, I think ...
but it would have the advantage of looking more 3-dimensional
from different angles, if you wanted to change the camera angle.

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