I don’t have data to back the following statements up, only personal first hand experience.
I think the potential to damage a 35 or 40W laser lense is low when lasering highly reflective and possibly polished materials such as stainless, copper, and brass.
I have in the past my job was to operate a 4.4kw (4400W) CO2 laser and while we usually tried to stay away from lasering (cutting) brass and copper, the few times we tried it we would apply a fogged/misted top coat of a proprietary oil to the sheet before placing it in the cutting chamber.
As it was attempts to cut copper with a laser proved troublesome enough to have the job reprogrammed to be performed on a punch (due to both the reflectivity (material reflecting instead of absorbing the energy) and inconsistent cut quality.
That being said we never managed to damage our laser or lenses from these attempts, which used orders of magnitude more power than the GF does.
My gut is that as long as you’re only attempting to ablate/blast off the etch coating, and perhaps do so with multiple lower powered passes that you’ll mitigate the hazards to the lens from any potential reflection.
I also intend to do this form of etch-resist erasure for metals work. I’ll post more when I finally get around to doing it.