The coating is what gives an optical lens its weather and scratch resistance whilst reducing reflection. Much like the previous cleaning process, lenses are racked and dipped into small heated baths of the required coating. This is to allow a full application and an even finish over the lens surface.
Optical lenses can be layered with up to nine different types of coating. Due to this, some baths are left for extended periods of time, allowing the coating to congeal. Lumps on the surface of coated lenses are the result of this, and can possibly lead to permanent surface defects when the coating hardening. To avoid this, the bath is intermittently emptied and filtered using a particle filter. Not only will this fix the issue of quality, but it will also reduce waste by reusing some of the coating that has already congealed.