Lots of work! I’ve used bleach for changing the color of stains but it never seems to get back to the same grain appearance. If it’s still structurally ok I’d call them character marks and varnish over. Trevor’s suggestion to use boiled linseed as a primer is good advise. You might be pleased with the look of that.
I’ve had good luck painting over a few coats of UV protected epoxy too, but I’m not going to suggest you do it. It’s your call. It’s a lot more work to keep the epoxy coat smooth.