I'm not so sure that washing is a great idea with these particular units.
The trouble with the late model Apple keyboards is the membrane. It's three layers of plastic, two conductive with an insulating layer in between, and the sides aren't sealed. Liquid seeps in and gets sucked in there and can't easily air dry.
You can carefully separate the layers and try to dry them off (easier said than done unless you have a nice, seriously absorbent cloth), but be careful of the points where the layers are welded together, the plastic is easily torn. Those weld points also have a tendency to want to hang onto the liquid
The next problem is that coffee in particular seems to be especially good at eating away those printed conductive traces. That and the wiping and bending action can easily break the traces. If you're lucky and it's not in too bad of a spot, you can manage an effective repair with plain old tin foil and cellophane tape, and lots of patience (don't even think of attempting a repair with anything hot). Don't worry too much about tightness, the keyboard when assembled will provide enough pressure.
At this point, $25-30 for a nice basic non-Apple keyboard starts to look like a bargain
