I have not done it, and am glad that I never will, but if I was faced with it I would try sanding
with the edge of a folded sheet of sandpaper. If the groove was wide enough, I'd fold the sandpaper around the edge of any flat object, such as thin plywood
. I'd experiment
with different grits to get the right finish in the groove, or possibly lower it with coarse paper and finish with finer. Have you got a place you can start that isn't too obvious?
Any powered solution to the problem that I can think of, such as using the edge of a grinding disk, would put the teak
itself at risk if you had a senior moment. Try that at your peril...