How do I clean the inside of a domestic air conditioner? Can I spray water to the coil (after the filters are off) or water will make coil to get rust?
The coil gets water (condensate from the air) on it all the time in use, so putting water on it should not affect it. How effective that will be in cleaning it is another issue, and "where else you get water" while spraying it could be a problem, but as to your question, "No, the coil is perfectly fine being wet, happens all the time."
There are commercial spray cans of cleaning liquid for evaporator coils. I have used some and it seemed to work , but I do not remember the name. Our coil lasted 22+ years before apparently dying . Any coils I have seen are aluminum so they do not "rust".If you have red rust , it is coming from the galvanized steel water collection pan .