Please let me know if you have any "secret recipe" for stubborn dirt.
Recently i acquired a recent multicoated lens. It is damaged and it was priced accordingly.
The seller told me that the front glass was damaged by a firecracker, which exploded close to the camera on New Year eve.
From the pictures i thought that it was impact damage, some small dots caused by tiny objects hitting the lens at high speed.
When i had the lens in my hands i realized that there is no missing material. The small dots are in relief!
I think that during the explosion of the firecracker, some molten particles hit the glass and got stuck to it!
At close inspection the dots look glass-like. Nothing comes out if scratched with a nail.
Normally i use isopropilic alcohol or ROR. I am not at home, and i don't have them at hand, but i gave the lens to a friend of mine, who will try with the alcohol and with a commercial lens cleaner.
I am almost sure that it won't be enough. Before trying to pry off the particles with warmth (hair dryer), i'm trying my luck on this forum. Maybe some member has an idea that could work...
Somebody gave me the advice to use WD-40 or Svitol (a locally made oily spray, used to loosen rusted screws), but i'm not sure it won't affect the modern multi coating.

I will try any suggested solvent ... the only limitation: it must not eat the coating.

ciao

Paolo