How to keep leaves clean - any tips or pro methods?

[Delila]

I've noticed that orchids that come from professional growers, and even from some hobby growers, have these lovely pristine clean leaves. They are perfect. Mine, however, especially those grown outdoors in shade, gradually develop a dust-like layer that looks a bit grungy.

I actually think it's a grey fungus rather than dirt. It grows best on the most shaded orchids - on their leaves and even over the outside of their pots.

Is there a special way to...
a) Prevent this?
b) Fix it once it's happened?

Thanks