Kane's face

DK - This is a most compelling topic! Indeed; the facehugger does not damage Kane's face. That is likely due to the fact that it did not want to risk killing its host and, thus, missing an opportunity to continue its terrifying life-cycle! Thank you ever so much for presenting this extremely interesting query! :)

No you didnt dk :)

This guy however no luck with helmets

Yeah i guess maybe we could have expected some marks.. but maybe the Face Hugger can control its Acid as far as how much to use to get through to the Face.

When Ash, Dallas and Ripley go to the autodoc to look for the facehugger we see a close up of Kane's face, there does appear to be some blisters on his skin.

My guess would be the thickness of the glass in his helmet saved his face from major damage.

Perhaps the facehugger can give a controlled burst of acid, just enough for it to melt through?

I've wondered it.I think the facehugger must have used a controlled amount,just enough to melt the helmet.Though in Alien 3 the dog show's up with burn Marks on It's face resembling the facehugger.

EDIT:Just noticed i pretty much said the same things BigDave said.Didn't mean to sound like a parrot lol.

Not the best example, but this illustrates what I meant. Seems like some spatter would have got on him. Done belaboring the point now.

It breaks a hole in the helmet, then secretes enough acid to weaken it before it slips inside.  No reason any acid should've gotten on him.