“I removed a rubber backed mat from my white floor in front of my kitchen sink, and the area under the mat has turned yellow. What will remove this stain?”
The short answer is nothing. It is likely a linoleum floor, which as a result of having moisture trapped under the mat has left this permanent yellowing. There is no way to remove this type of stain. The best solution is yet another mat to hide the stain or to replace the flooring. A jute or natural backed mat will permit moisture to escape and will not cause further damage.
