Countertop refinishing

Submitted to Ask the Makers

I have a solid cherry countertop that I am finishing in place. "from a distance" it looks ok but the reason I started down this path is there are a few spots near the sink where water got between the boards. The spots are about the width of a pin. I've sanded multiple times and then thought I might be able to use a wood filler product from General Finishes to hide the spots but cannot find the wood filler. Does anyone have an experience with this kind of problem. I hesitate to make the openings larger and I'm not sure how much more I would need to sand to eliminate them. I've tried an application of hydrogen peroxide and that helped some.