Superhero

Anyone got some ideas as to how to unstick a stuck valve retainer? Someone I know recommended taking a screwdriver and placing it against the valve stem under the retainer, then lightly whacking it a few times with a hammer. I did this and ended up with some small gouges in the valve stem. It also didn't work at all.

I knew that was a bad idea before I did it; I should have known better. Is it possible to polish a valve stem while it is still in the head, which is still attached to the engine? The gouges don't have any sharp burrs or edges, but I am sure it'll still wear those seals quickly.

Thanks