Hi chaps , i have been doing a job restoring upvc back to looking new , now although i have done 9 flats so far with no probs i have 3 bottom flats with the cill which has turned yellow , now i have been using viro-sol at 25;1 ratio , then at about 12;1 ratio , but the yellow is still there . The tenants are fine with it but im not,as anyone got any other ideas , cheers in advance