It has a thickness, so it could keep the key from fitting in the hole. You can try and scrape it off if you see that it doesn't work. Don't bother putting any on the teeth, it'd wear away in no time, nor in the grooves, it would most certainly obstruct them.
I used nail polish as paint and varnish for small objects a couple of times and liked the result, so don't hesitate to try. If the result is bad, it's not hard to remove.
