I had a lot of rust on my Wipacs today but after using "Bar Keepers Friend" on them they are back to their former glory! My rust was spotted all over the back of the lights but quite dense at the base, the stuff on the back has been totally removed and the majority of it at the base camw off:
"The best professional powdered cleanser since 1882. The unique formula in Bar Keepers Friend gently removes stains other cleansers can't. It will even remove rust, and is great for cleaning stainless steel."
Bar keepers friend website (American site but the product is available in the UK)
Think I got mine in John Lewis, give it a try as I have been using it since I installed my lights nearly a year ago