Blight!

[Rachiered]

Hi,
I was wondering if anyone could advise me on potato blight. I have bought some bordeaux mixture, which I have used a few times over the last few months, but due to the constant rain, I cannot spray my spuds, and they're starting to look really poorly.
Once sprayed, how long does it need to stay dry on the leaves, before the rain washes it off? Will it carry on working even after it has been rained on?
Please help.
Thanks

[Two_Sheds]

How to use: Mix the Bordeaux mixture with water and apply as a spray to foliage as directed on the instructions on the pack. Ensure both sides of the foliage are adequately wetted with a fine coating of spray without drenching. Spraying requirements vary depending on the plants to be treated. (See pack for information).