My UV filter seems to have some slight smudges which I can't remove by wiping with lens fibre cloth. The filter came with a 2nd hand lens I bought. I have never drop the filter before and l always blow and wipe the filter after use. Why will there be unremovable smudges? Is there anyway I can remove the smudges on the filter? I am thinking of using lens cleaning solution on my fibre cloth to try to see whether i can wipe away the smudges. For cleaning of lens or filters, issit advisable to just use the fibre cloth or use fibre cloth in conjunction with lens cleaning solution?