Hard facing and machinable don't belong in the same sentence
However, that never stopped me from doing it anyways. I always used Greenleaf WG300 ceramics to knock the humps off. Pretty amazing ceramic, you'd swear when using it that what you're cutting maybe isn't all that hard, then you stroke it with an old file, which skates off it
I used a tool steel welding rod, which is not as hard as the super alloy hard faces, but still hard. Pretty expensive rod, and wow, a 50" ring sounds like a helluva chore to face all over. Might take hundreds of dollars worth of rods.
There is no such thing as a moderately hard deposit: it goes on very hard, and then maybe you can temper it back a bit if you have a furnace to do it with.
How heavy is this ring going to be? It is going to warp horribly if welded only on one side, so you might have to weld both sides to keep it decently flat.
How about getting a ring cut from AR plate: abrasion resistant plate. It comes in various flavors, I think AR450 (brinell 450) is pretty decently hard, and can be used as is, or machined with carbide, and drilled slowly with HSS.