Rain-fast 
Some chemicals, especially pesticides, which are applied to a surface might be called rain-fast. This means that once applied they would be less likely to be washed off the leaves of vegetation after normal levels of rainfall.
Rain-fast
Some chemicals, especially pesticides, which are applied to a surface might be called rain-fast. This means that once applied they would be less likely to be washed off the leaves of vegetation after normal levels of rainfall.