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?
Is it just a soapy solution?
Does it need to be brushed into the fins with a soft bristled brush?

Thanks,
Walter