Deoxit is the classic chemical solution. Try not to breathe it.
Exercise all knobs thoroughly. Twist them back and forth. Move the switches through their range 100x each. Gently but firmly.
Once you've gone through it once, hook up inputs and outputs, and AT LOW VOLUME, run those faders up and down. Clean yet? With most dirty level knobs, you can find the chunks of trouble and work them out with extra elbow grease. GENTLY but repetitively! Several hundred scrubs is not unusual to fully clean a chunk off of there.
If you can't get it to adjust cleanly, then do like Dsatz, and get it set, and leave it alone.
Even if you get it clean enough to run, check it before you take it out next time. It's very possible that the dirt in the faders creeps right back in as soon as you turn around.