As others have mentioned here on the forums, keep an eye on the rubber feet on your NeXT hardware, as they can turn to liquid goo. The sound box on the left was actually stored upside down and the goo still ran like wet paint. The one on the right is just starting to go. NeXTstation feet can liquefy too, as can monitor roller feet. Even the feet on PLI Super Floppy drives can deteriorate.
The goo can removed with a lot of elbow grease, a plastic scraper and some mild cleaner. There's two sided tape under the rubber pad, so if you can get under that then the pad can be peeled off in one piece. As always, test any cleaner in an inconspicuous spot before applying it all over!
isopropyl alcohol usually works well also
I recently marred the finish on the top of a NeXT nob-ADB keyboard with isopropyl alcohol, so care needs to be taken where it's used. Unfortunately I found this thread (
https://www.nextcomputers.org/forums/index.php?topic=4382) after the fact. It's a good idea to test any cleaner in an inconspicuous area first.