There are pressure washer attachments that basically turn the unit into a media blaster. You can use whatever substrate you prefer and that ranges from sawdust for polishing to coarse grit sand for blasting scaly ferrous metals.
I would think that the fastest easiest method would be to blast with a gentle media like soda with a pressure washer.
Can I use ARMEX® with my pressure washer?Yes. There are media injection devices available to introduce ARMEX® into the water
stream of your pressure washer These attachments are available through the authorized ARMEX® distributor network.What is ARMEX®?ARMEX® is a baking soda based (sodium bicarbonate, NaHCO3) media manufactured by the makers of ARM & HAMMER® Products. There are over a dozen formulations designed to meet specific surface preparation requirements.ARMEX® - Baking Soda Abrasives: News and Information - FAQs
For the stubborn areas try this ---Nyalox, the original abrasive nylon brushes have been redesigned to improve performance over a broader range of uses. NYALOX is the only nylon brush line available in three grits and bristle diameters.