What I’ve done is:-
1. invest in a good palm sander such as a Makita. The one I’ve gothas about a 4″ square pad, you can buy rolls of sandpaper at good buider’s merchants, and simply cut off a square to fit. Saves a lot of money rather than buying specially cut sandpaper squares/traiangles.
2. Get a triangular paint scraper and a file. File an edge and keep it sharp!
The scraper lets you get into the corners fairly easily, and the palm sander will allow you to palm sand right up to the edge.
Rustins floor ‘varnish’ is the stuff to use, I recommend to go for a matt or satin finish, the gloss is very shiny!
After all that you need to put some sort of tread on the actual star where you walk to cut down sound and reduce wear on the wood. I’ve used a cheap corduroy and cut the tread to sit on the actual flat of the stair, floor tacking ( or stapling ) it into place and rolling it around the bull nose of the stair. This gives a protected and sound proofed actual tread, but allows the wood of the riser ( upright ) of the stairs to be seen.
It takes a while, but you only need to do it once!
best of luck!!!!
Q