Rain-x is a water repelling liquid used for vehicle windscreen, mirrors and shower screen. Spectacles too to prevent from water droplets and fogging similar to saliva.
While I have not encounted any problem on any glass surface, I have not tried on camera lens which is why I previous stated to use on plain clear glass filters. The only issue is surface must be cleaned prior application else the stain will stay. Tissue cannot be use as it will leave streaks, cloth is the best.
Rain-X is alcohol in content so plastic and paint beware. Rain-x is not so popular for front windscreen bec the rain wipers encounter high surface resistance when the water did not spread enough.