Interesting comments from above. I used Opti-seal on bathroom mirrors yesterday and tried it on one that I did not first clean. Bathroom mirrors tend to get some tooth brushing splash back if you use a Sonicare or similar device. Opti seal did NOT remove that residue. If I clean bathroom window first with something, Opti seal is great. I've used Opti Seal on exterior windows of my house for a while now with great results, but only when I first clean with Opti Power Clean, or DG glass polish. For car exterior windows, I use WG L S with great results.