Hey, Robin, 'Sup?
I'm not surprised it would take that long. It's not a modular board so to get at the insides you have to take off every knob, nut and washer on the thing so you can remove the main board and clean it. And you have to figure out how to get it apart without breaking something or letting the main board flex enough to crack or disrupt any of thirty-five quadrillion surface mount tracks or components. Probably four of those hours are for dis/reassembly, and those guys know what they are doing. Taking one of these apart is a pain in the ass, but with patience you can do it.
Cion, the best thing is to be organized and give yourself plenty of room. This is likely to take a couple of days if you've never done it. I've never taken one of those apart, but have done similar desks. Get a bunch of containers to hold knobs, screws, washers, nuts, etc. and
label them all, especially the ones that hold the damn thing together. Make seperate containers for every different type of knob. Make a container for all the screws on the bottom, a seperate one for the screws on the back. You get the point. Leave nothing to chance or your memory. I have even made diagrams on paper and taped the screws to them where the screws are supposed to go. Never say "Oh, I'll remember where that goes". You won't remember five minutes later, never mind three days later. There is a saying- "The dullest pencil remembers more than the sharpest mind". Most of the work is going to be taking it apart and reassembling the thing. That alone will take you many hours. You will understand why they wanted to charge you $95 an hour
.
I'm gonna recommend the static strap. There aren't digital components to blow, but there are plenty of ICs. It's a surface mount board, and if you blow something you will not be able to repair it, if you can even find the problem. Off to the shop it goes.
Try to take the chassis apart before you take off the knobs or the allen screws that hold the main board, to see inside and make a plan of attack. That way you can turn the thing and move it without risking damage to that all important pc board. I've had it go that the best way was to leave it bare and remove the board, or seen that it is best to reassemble to a point. Be careful- once you get the sides off it will be very flexible. Start by taking off the back if you can and look inside. Something soft to rest the mixer on is nice. Once you are ready to remove the board I'v had success supporting the board from under on a blanket so it won't drop, removign the allen screws, and lifting the chassis off
carefully. Also had it where you stand it on end and kind of flop it toward you. 24 channels is a bit big for that, though.
Mark all ribbon cables and their connectors- magic marker works well. The ribbon cables connect all the boards, in/out connectors, and meter bridge. Gotta get those back in the right places. If you want to clean behind the back panel, it's the same. Pull off every nut& washer to release the boards. The jacks & mic inputs should be OK with some deoxit. Spray it on the appropriate connector and insert/remove a few times. DO NOT just spray deoxit in the jacks. Some electronic things melt with a lot of deoxit on them.
Robin had some great suggestions. If you are gonna vacuum, use a
very soft tip and definitely a static strap. Try to touch actually touch the board as little as possible with the tip. Surface mount components and traces are very delicate. Use moving contact lubricant cleaner and lube on faders, not deoxit.
If the pots on top have nuts holding them, get the right sized hollow shaft nutdriver. Same for 1/4" jacks. You will save yourself much time and ass-pain.
You will need a small allen wrench to remove the support screws which hold the boards to the top of the mixer. Get two or three. These tend to be very tight and it is likely you will snap an allen wrench or two before you are done. Be prepared to use the vise grips, also. Those screws round out very easily. If you can't get them out with vise grips it's drillin' time
. Or try a craftsman screw remover. Then you gotta figure out where to get the damn screws.
Have small thin screwdriver for prying ribbon connectors apart, and just have every kind of screwdriver there is anyway. One of those extendable magnets from Radio Shack is handy for retreiving dropped screws.
I could probly think of more stuff but i'm tired. Good luck.
p.s. the biggest problems with dust are heat build-up, noisy faders, and bad connections in jacks. It can also trap things which corrode your gear Down and dirty do all the jacks and faders, take off any easily removable panels, and get what dust and dirt you can. A thorough cleaning is best, but....
.
you're surly making this job seem VERY hard. [Not that I think it will be easy]...wow, you kind of made me wish I'd let DMB do the job, but hey...for 
that can crack, and how could they crack?... 
