Assuming it is marks from removed oxidation,..using a cleaner like Purple Power applied full strength through a sprayer ,..then brushing and rinsing small sections at a time would work. I realize saying "soft" brush is a relative term,..But don't use too soft a brush, you want it to be somewhat aggressive,....use a brush typical to the truck wash industry type truck wash brush,..that's likely what you meant anyhow.
You can also spray the Purple Power on a section and using a wide fan pattern nozzle and about 800-1000 PSI and evenly work the oxidation off,..this gets tougher the higher you go.
Removing oxidation is much easier when the temperatures are very warm,..the cleaner will be more aggressive,..but you also have to be more on your toes for it not to be drying.
You can sometimes blend it together with just a brush and water,..Oxidation on darker colors is always tougher to do,..(Greens, Blues, Reds, Dark Grays, Brown) Especially blue,..I HATE blue siding,Ha,Ha,..
These are just loose suggestions,..as it can be tough to determine from my recliner,..even with pictures sometimes.
*Two-Stepping is also an option,.again though depending on what you're dealing with.
Is the whole house this way or just the most UV exposed side(s)
Jeff