What kind of pads are they?

Shimano pads seem to melt in the rain - with the black grit getting all over the braking surface resulting in that gritty feeling and noise. Kool Stop pads, in addition to stopping the bike better, don`t melt when wet, last longer and don`t pick up shards nearly as often.

You could also go over the rim braking surface lightly with a fine abrasive rubber block like this - http://www.amazon.com/SandFlex-Flexi.../dp/B007R2LPBW