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I don’t think anyone is suggesting you take a hose to your indoor units. How on Earth you would contain the splashback is anyone’s guess.
Use your hose on the outdoor unit. I use compressed air on the indoor ones, like OP. You can buy the cleaning foam stuff, too. Probably from whoever made your split system, in fact. I’ve never found it to be necessary, though.
Yeah. Just prepare for dust. Wear a mask, open a window…