When I clean my attic a/c coil do I need to clean both sides

Usually the cleaner is applied to the air in side since most of the dirt is there but it is always preferred to use enough cleaner to penetrate the entire coil since dirt gets trapped in the inner part of the coil also. Most approved coil cleaners will not require a flush since when the unit is operating the condensate from the coil will wash off the cleaner and dirt down the drain.