"Walter Cohen" wrote
Several people have told me that when they have their central a/c units
serviced the technician sprays a liquid on to the fins of the outside a/c
compressor unit and then they wash it off with a hose.

What is this liquid and what should I be using, as I want to also do
this?

Unless you've got greasy build up (and/or a lot of 'baked on' dirt) then
just flush the condenser from the inside out with a garden hose.

If you insist on using a cleaner I'd avoid any of the common hydrofluoric
acid-based coil cleaners. Here is a safer alternative:
http://www.patriot-supply.com/produc...item.cfm/55541

HTH.