Ah, the Roland JD800. When I first saw a picture of this beast years ago online, I was in love. After fiddling with many rackmount and menu-driven synths that I got for cheap in highschool, seeing something with this many buttons and faders on it had me infatuated with the amount of immediate control one synth could have. Flash forward a decade, and I actually have the means to purchase one off eBay, although it has some issues of course which dropped it into my price range. The seller shipped it in a flight case which was pretty cool too, which added to the giddy-kid demeanor in which I eagerly cracked it open as soon as I brought it inside my home.
Removing and reattaching the keys can be a little tricky, so just take your time and don't move too quickly. There's a skinny black strip (not pictured) that holds the keys in on the underside of the keybed that must be removed first. Try not to bend it too much, or it can crease. White privilege exists even in keyboards, so you must remove the surrounding white keys first before you can get to the black keys. Start by pushing the key down, and then pushing up on the key from the bottom side (the most rear part of the key, pictured near the bottom of the picture). You may have to push/wiggle it a little, and then it is free to move. If it gets stuck, then it is most likely the metal "spring" caught somewhere, so don't force it.
As many other how to's and forums say, lye or sodium hydroxide drain opener is the go-to solution to rid of the red glue from the keys and keyweights. Lye won't harm the plastic or metal from the keys or weights, but it WILL attack the yucky red goo and dissolve it in the solution. Also, of course be careful and read the safety instructions - lye can cause chemical burns, blindness, and breathing problems if handled incorrectly. I used some beaded Pure Lye Drain Opener from Amazon, mixed with water, to soak the keys in little cheap plastic containers I got from Home Depot for like $2. I soaked each batch for about 1-2 hours, then drained it and added fresh water and lye (simply so I could check and gauge how well it was working) and after a doing 2 soaks for about 2 hours each, 99% of the glue was gone. After that, I carefully rinsed and soaked them for an hour or so, in some soapy water to get any lye stank off.
While it was all soaking, I worked away at the glue stuck to the metal chassis below the keys, where the weights had fallen onto. Using lye there wasn't really an option, safety-wise, so I explored some other options. To get chunks off, using an X-Acto blade carefully to scrape it off, and even pliers to just yank it off if the chunk was big enough (its sort of stringy so that didn't always work). Once you get the glue chunks off, I found that 99% Isapropyl Alcohol, paper towels, and my finger nails worked best. I'd take a small strip of paper towel, get it a little wet with IPA, then scrap away with the paper towel in between my nails and the glue. This might not work for everyone, but I was already planning on trimming my nails soon anyway (an alternative might be a flat paint scraper) The reason I chose my nails though, was so I wouldn't scratch the metal chassis underneath, which anything metal or sharp would do so. My JD800 had glue ALL OVER the chassis, so I was being a little protective and went with the slow and safe route I guess. After maybe 45 minutes to 1 hour, I got about 99% of the glue off the chassis, so I was happy.
OK! So the glue was gone, I put the keys back in, and eagerly powered her up. To my dismay, nearly all the keyboard issues were still there except the keys that stuck to each other. I had to go back in and remove all the keys again, so I could clean the contact strips with IPA. They didn't look too dirty, but I guess even the slightest dust/dirt can block their path. DON'T clean the grey rubber part of the contact strip, as it can ruin it. Just wipe the black part of the strip, and the area of PCB, gently and don't overdo it or you'll remove all the carbon off it! The JD800 also has a nice test mode for a few things, including velocity sensitivity on each key, which makes this process easier so your ears don't fool you. With velocity and dead keys now fixed, I still had a couple keys that would stick for a second after you released them, which I found was caused by red glue that had gotten on the felt strip below the keys. There are two felt strips, a top and bottom red one. I took some flush nippers and cut a bit of the felt out carefully.
While I was in there I also popped in a fresh CR2032 with ease, and put it all back together. On startup, it made me do a reset which loaded fresh factory presets into all the banks because of the battery switch. All keys now worked and I had a nice JD800 ready to receive some loving!