Years ago we bought special child proof door knobs so that my youngest child couldn’t get into certain rooms. The doorknobs were made of plastic with two rubber buttons you have to push to open the door. After doing that you turn the knob and it opens like a normal door. Just recently we took off all of the covers. All of the doors that the knobs were moved off of are now very sticky because the plastic caused a green substance to form. I guess it’s some sort of oxidation. I’m having a heck of a time getting it to come off. I’d put the covers back on except they broken when I removed them. Any suggestions? Has this ever happened to you?