A coating is a thin film, often softer than the glass under it. It fails for mechanical reasons long before it fails for chemical ones: a fibre dragged across the surface, a dry edge, a wipe that picked up a particle and then travelled. The solvent is usually blamed because it is the visible variable.
Drag is the enemy
Friction between a dry fibre and a coating is enough to leave a sleeker mark that shows only under a point source. The control is lubrication: the surface should be wet enough that the wipe glides, and the wipe should be wet enough that it does not dry mid-pass. A dry wipe on a dry lens is a polishing operation, not a cleaning one.
Technique matters as much as the cloth. One pass in one direction, from the centre outward, with the wipe turned or replaced before the return. A back-and-forth motion drags the loosened particle back across the coated area.
Solvent choice
Most optics houses use a sequence rather than a single solvent: a solvent to lift the soil, then a rinse to remove the solvent, then a dry pass or an evaporation step. The exact chemistry depends on the coating and the soil; the principle is that the last liquid on the surface should be the purest one.
Water spots are a residue problem, not a drying problem. If a spot appears, the water carried something. Use purified water and a wipe that releases it evenly, and do not try to dry faster by using more alcohol on the final pass.
The wipe, step by step
A soft, low-lint wipe with a sealed edge is the default for coated surfaces. Cut edges shed, and a single fibre under a wipe is a scratch waiting to happen. For a small lens, a folded wipe used with a pair of tweezers is more controllable than a finger wrapped in cloth.
For high-volume production, pre-wetted wipes remove the solvent-dose variable and the bottle contamination risk. The trade-off is the open life and the cost per sheet, which is usually justified by the reduction in rework.
| Step | Consumable | Watch for |
|---|---|---|
| Lift soil | Soft sealed-edge wipe, solvent | Too little liquid: drag |
| Rinse | Fresh wipe, purified solvent | Reusing the first wipe |
| Dry | Dry wipe or controlled evaporation | Water spots, fibres |
| Inspect | Point source, loupe | Sleeks under oblique light |
When a scratch is not a scratch
Some marks that look like coating damage are residue films or sleeks that can be removed with a proper wet pass. Before scrapping a part, inspect under oblique light and try a controlled re-clean on a small area. If the mark remains after a wet pass and a rinse, it is mechanical.
Record the position and the direction of the mark. A scratch that follows the wipe path is a technique problem; a scratch that follows the part geometry is a handling problem.
Training the pass
Teach the pass, not the product. One direction, light pressure, a fresh face for the return, and a stop when the wipe feels dry. A new operator who learns the motion on a scrap lens will reproduce it on a coated one.
Assess with a witness slide: clean it with the same technique and inspect under a point source. The slide shows the film and the fibres that the eye misses on a curved surface.
Can microfibre be used on coated optics?
Yes, if it is clean, soft and wet enough. A contaminated microfibre is worse than a clean paper because it holds the particle against the surface. Use a fresh wipe and a sealed edge.
Is a lens tissue better than a wipe?
For a small, delicate surface, a folded tissue gives control. For a larger area, a wipe with a sealed edge is more consistent. The technique matters more than the format.
How do we know the wipe did not leave a film?
Inspect under a point source at an oblique angle after the surface has dried. A film shows as a haze; a particle shows as a point. If in doubt, run a solvent blank on a witness slide.
