I have quite a few len filters. I bought them about 10 years ago. Recently, I used them to stop down shutter speed because i shoot wide open. One of the filters, I have never use before and keep in the plaster holder. I tried to clean it with spectacle cloth but to no vain. Is there any solution i can use to clean them. i have 2 polarisers. if the solution get in between the 2 glasses, will it make them worse? Do i need to keep my filters in the dry box? Please advise.