lightening a valve... without a lathe

[JonE]

I have a scrap valve to try reducing the throat area above the contact/guide. I attempted something with a new Dormer bit in a dremel, but they are too small and create a slightly pocked surface. I've also tried a much larger burr in a Makita, which copes nicely with the curved areas but is no use on a flat sided stem. All these whilst rotating a valve in the pillar drill.
I realise that a lathe is the only way to do it professionally, but that isn't an option, so what other ways can I improve with what I've got? I wondered about a file or stone against the valve stem.