Bob,
I just had another thought on your problem: If the run-off is chalk residue from the aluminum siding, a lot of times just plain P/W at about 1500 PSI will remove it. If not then you may want to try the detergent that you use on a regular basis, before spending the money on Prosoco 600 Detergent.
I always carry different solutions on my truck because you never know what you come up against. I have also developed a cross reference (spread sheet) of stain types to solutions.
Most times (if it's a smaller area of stain) I start out with the simplest method (KISS) and try nothing but P/W starting with low pressure then increase pressure. When it's a larger area
of stain or if I short on time, then I go for the heavy stuff.
Regards,