During the period of time from 1970to 1990, large quantities of polymeric materials were applied to painted artifacts in museums or in situ to wall painting to stabilize and protect pigment layers. At that time, about 40years ago, both suitable conservation materials and application techniques were undeveloped and limited. Now those aged protecting organic layers, such as acrylic polymer and Sanjia resin, have caused new problems for conservation. Challenges include gloss, yellowing, craquelure, flaking, etc. This paper lists a few cases that illustrate how degraded polymers have destroyed wall paintings and how they have had negative effects on wall paintings. Then, the concept of “degradation” was defined, illustrated and categorized into two groups. A situation falling into either category could be considered “degraded” protecting layer and must be removed for longterm protection of the mural paintings. Finally, effectiveness and longtime effect of gel cleaning and microemulsion cleaning techniques were discussed.