Calcium salts sometimes precipitate on the leaves of bonsai in hard water areas. Rain is great because it's soft water and the pH is usually just a touch acidic. If you are in a pinch before a show can't come by any rainfall, you can polish the leaves with a little anti-stress 2000. Now that it's 2002 (nearly) you'd think they'd have something better available.
I've been trying to find a picture that shows the product label I get in the US, but I keep seeing the Aussie concoction, that has a very similar description. It's a milky white solution that you spray on the leaves but it dries glossy and clear. We use it before shows sometimes. The type of stuff the plant handlers use to polish fig tree leaves, leaving them shiny. No actual buffing necessary.
It claims to be an 'acrylic copolymer,' but it looks and smells like Elmers glue to me. Don't ask how it tastes
Here's the Australian link. I'll keep an eye out for the other
http://www.antistress.com/explain.html
Regards,
Matt