In the past, commercial contracts have wanted me to bleach paintwork, slates, etc. At that time I had a separate 50 litre trolley which I filled with the chemical mix, used it as standard and then flushed through with pure water afterwards. If you don't have a separate trolley system, you can fill a 25 litre jerry can with whatever mixture you want to use (detergent, TFR, etc) and then temporarily put the feed from your pump into the jerry can and then back to your main tank when you finish.