It is amazing how many folks ignore their AC units season after season and still expect them to function as normal.
The best thing you can do for them each season is clean the coil.
BUT if you touch them with a brush or anything like it you will ruin the coil.
Just spray, let sit for a few minutes and rinse with a lazy stray from a low pressure garden hose..
This is the stuff to use and at an amazing price. One can per AC unit does the job with a couple application and rinses.
Be Sure And Wear Safety Glasses.