Whenever I've had a scratch to repair , regardless of if it's on a cameras prism or the bodywork of my van , I've just dealt with the scratch and not removed everything to start again .
But I think you know this now ....
When I had a prism off a Bronica ETRSi that had clearly been dropped , as the top was dented ( when I bought it very very cheap ) and had removed a bit of the coating that was visible in the viewfinder.
I covered the missing bit of black with a bit of black ink and it became barely noticeable.
I feel that black paint would have done a better job , but my issue might have been damaged to the glass .
As could yours if you've sanded too hard .
Be sure to mask off well the areas that have to remain clear .
EDIT ; I might be misremembering , this was a good few years ago .
I feel like I might have used one of those silver paint type marker pens , thinking that the outer prism would be better off silvered / aluminium coated .
So instead of looking at a black stripe where the original coating had come off , it's still silver .
Not as bright as the rest but not distracting, and not a large enough area to make a difference.
When you say you've sanded "it" off , do you mean your down to clear glass , or a silvered coating ?