During the period of time from 1970 Lo 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 40 years 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 long – term protection of the mural paintings. Finally, effectiveness and long - time effect of gel cleaning and micro – emulsion cleaning techniques were discussed.