Don’t use alcohol in an ultrasonic machine, especially if you are inside. (Risk of fire/explosion doe to the atomizing of the alcohol!)
Use a commercially available PCB flux cleaner instead. This stuff for example, “Elmasonic 800 0101 Elma TEC Clean A1 Ultrasonic Cleaner Solution Concentrate for Electronics and Optics- Powerful Cleaning Fluid for Industrial Use”, is about $265 per 2.6 gallons. It a concentrate but that’s still pretty pricy!
That said, if you clean a board ultrasonically that has potentiometers on it, you will have to properly relube them as that will be history also.
Is a brush (like the ones Alex sells) and alcohol looking better?