Aluminum siidng can be wicked depending upon age, quality and how it has been maintained. More often than not it will come out blotchy (especially with an X-Jet). I don't know what you mean by big job but I usually charge double or triple on a house with AS as I apply my chemicals with a shurflo then use a 4010 on the entire house being sure to keep the nozzle the same distance from the house all the way around...not easy! I would definitely have the customer sign a waiver and inform them of possiblem undesirable results.