This is more like a slim guide for people looking for an idea what they have to expect. Not every screw, cable or step i did is here documented (did the pictures mostly for own reference), but its mostly obvious what you have to remove.
Disclaimer: Do not do this by your own, if you never did some small repairs at electronics. I have to say i am not a trained repair guy, i just started couple of years ago with repairing electronics and did many mistakes back than. This repair was for me a straight forward process and easy/mediocre project, without having a service manual.
Repairability: Some words regarding the repairability of the Mixer and a lot of people say its not possible. I repaired a couple of things at this point. Started with a used Pioneer DDJ SZ, a flagship controller, which was way more challenging to get to the broken parts than this repair. Yes it would be good to have some easy access to the faders, but it is doable and while it needs time, it is a pretty straight forward process. Keep in mind the price difference to other mixers which have this feature, but are way more expensive.
Original Problem: I noticed a drop of the sound quality on several faders. Also faders where not at the same volume level while playing the same track and the gain nope was also the same. (maybe the spillage did something to the sound as well → clean it as well?) → after replacing the faders it works perfect again
Parts: You get the parts from several sites, just search for it. X1800 and X1850 seems to be the same faders. Here is one link where you find the number: Denon Pro X1800 PRIME Spare Parts - Denon Audio and DJ Parts
I ordered faders but the right parts number, and everything works, but i can feel that the faders are cheaper than the original. Noticed it after they were soldered, so i stick with it. Maybe you can look for better ones.
Time effort: around 3-4 hours
Difficulty: mediocre, just be sure you don’t break the cables
Cables: the flat cables have some black locking system you have to slightly open to remove the cable. Also be sure you have the locksytem open when you put it back in and than push the black locks back in to secure the cables.
Tips:
1 Spillage: if you ever spilled something (as the owner of this before me did) buy a very soft toothbrush and some Isopropylalkohol to clean the electronics, while you are in there. (make sure you read guides how to clean water damage on electronics)
2 Faders: replace all 4 faders at once. While only 2-3 are seemed not to work i just replaced all 4 to not have to repeat the repair.
3 Clean: there is nothing better than disassemble everything and than be able to clean it. Honestly, the front panel can be removed so easy, its the best way to clean it without going through the gaps between the pods.
4 New fader check: if you bring the soldering part to a shop, let them check the faders that they really work.
5 Document process with pictures: What helps me is to make many pictures of every cable to have an easy way to identify which screw, cable or what so ever finds it way back to the original place.
6 Screw bags: Use some plastic bags or boxes to separate all the screws. Like the new lego sets with its numbers, its easier to put everything back and don’t lose a screw. Please just do it, its worth the effort 
7 Faders: when you unscrew the most things lay something under the mixer so the fader pins don’t touch the ground. Especially after the repair they would be damaged if they hold the weight of the mixer while you repair it. Could be easily bend and broke)
Lets start:
Unscrew everything from the Panels:
The back panel has a screw and attached cable behind the panel:
Be gentle and be sure all screws are removed. The outputs are easy to damage.
You can remove the front caps and the screws underneath now or later. I cleaned the removed frontpanel and underneath it. The 6 mm cue output for your headphones has to be removed as well.
To remove the PCB you have to unscrew everything as shown:
Mostly 10 MM
Now we have the naked X1850:
Start by unplug in all cables you can already reach (not all can yet be reached, so be careful when disassemble every PCB)
We start with unscrewing the higher panels on the left, and than go to the second layer until it looks like this. Three parts should now be removed. The process is pretty straight forward, just unscrew everything, as well as the golden higher screws that are need for the first pcb layer.
The three Stacked PCBs are removed, now start with the power supply and the right PCB.
I marked which cable where in which position:
Don’t forget to put out that cable. Maybe use a something flat to push it out:
One side removed now the power supply
I already unscrewed most, now its time to put out all screws and the the small cables on the left. (if not happened, you have to unscrew now everything of the front pods):
If everything is now lose you can get your board (when you get the PCB out, make sure the plastic body lays down, there are some small lose part like buttons that can fall out). Also make sure the connection to the crossfader is out (easiest way, after you removed the main PCB):
Now have your PCB with the faders:
Now you can solder it by yourself or look for an electronics repair shop near you. Since my soldering is decent, but not nearly as good as a professional, i tend to give this last part to the shop.
Now do everything backwards.
I hope this helps to get your gear back into the booth.
I have to say i hope there will be an updated, but since i am a heavy user of Soundswitch, i really love to have the standalone function that this mixer provides.
Greets, Milian Akeem