Inspiron 7000 (7347) get dirty & stained in the palm rest (SOLVED)

I know this post sounds weird. Just sharing that maybe somebody out there experienced the same.

After 6 months of all day usage my inspiron 7347 get dirty, yellowish stain in the palm rest. I noticed it when it's placed under cool white light. It is ugly, and i keep thinking that it will get more yellow as it ages under my palms.

The material used in 7347 is plastic with aluminum like finish in a rubbery matte feel. 

So I decided to give it a try, using soft sponge, dish washer soap to keep the sponge foamy, but not soaked wet. I rubbed on the palm rest, and the stain is gone.

Hope this helps to make your Inspiron 7000 series last longer.