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Depends on how bad it is the only acceptable cleaner is a non acidic coil cleaner.If they are too corroded, this may not even help.Are they to the point they flake if you rub your finger across fins?
You could try the alka brite or Nu cal designed for this..
You may want to clean a couple of times to remove any of the residue from products you previously used also.Any residue left can effect the ability of the coil to give up heat.
AS for a business.I see no reason one could not start a coil cleaning business just as some have successfully started business of just changing filters.
Most reputable air conditioning companies do this as part of annual maintenance.
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