I have heard a steel fist in a velvet glove-- good old maggie (thatcher) was called that early on in her career, but it was old when used to describe her.

was it english? or is it from some other language, in which case, the idiom might not make as much sense.. but it does capture the idea, that if you have the strenght, (steel fist) you don't have to get rough-- you can cover the fist in velvet, but it remains just as hard, and unyeilding..

a anvil is no less hard if you hit it with a hammer made of feathers.. -- the comment sounds strange to me.. it almost makes sense... but not quite.. but i expect it could...

(an other culture might look at a steel fist and say, whoa! who has a steel fist? )

EDIT: steel fist and velvet glove might give fewer hits, but one of the oldest is a PDF file about Maggie.
No doubt, iron fist is also used, but steel and iron are not only different metal, but invoke for me different images..

iron seems heavier, coarser and cruder
steel, is sleek, shinier, flexible, lighter but stronger than iron-- in a battle of iron and steel, i'd want steel on my side..

my other obsession