Oxidation needs to be emulsified, then it can be removed with slight pressure (300psi) or by brushing then rinsed away. Potasium Hydroxide or any Butyl based cleaner works well for freeing the oxidation. Because this takes extra cleaners, steps and more time, we charge a higher rate. This is considerd vinyl restoration versus simple cleaning away the mold/dirt. Oxidation isn't a CLEANING problem but a CONDITION problem. Customers need to be aware of the difference upfront. This is the main but not ONLY reason we use 100psi or less to clean vinyl. The other methods will disturb that oxidation and leave you with having to try and blend it all together all while only collecting a check for simple cleaning.
Hore that helps with the issue of oxidized vinyl. Aluminum is a bit of a case by case story.