Furniture, Wood and Cabinetry Finishing - Harwood Stain - how to remove

I recently removed carpet from stairs that are unfinished hardwood - it appeaars where the nails were in the carpet caused small bacl rings on the wood - I tried sanding them off but it didnt work completely. How do I remove them without damagain the wood? I do plan to finish the stais in a light stain will the stain cover these marks?

Also -on the stairs nears the side of the tread there is remnants of dark stain from the trim board that runs up the stairs - I cant get the sander in close enough to remove them - how do I get rid of them as well.

Any suggestions iare appreciated!!

Oxalic acid (commonly known as wood bleach) should remove most, if not all of the rings around the nails.

Hand sanding would seem to be the answer to the stain you can't reach - another alternative is KWIK paint/varnish remover. I've had some luck removing stain with this product.