For anyone who may be interested, I had the same problem with a couple of keys on the left hand section of the keyboard too. I'd bought a white bath enamel repair kit for a repair on my bath. The kit comes in 2 parts, the white enamel and hardener.
l decided to give it a try as it's meant for acrylic baths too. Mix a small amount together and place a couple of small blobs onto the indent. Smooth it over the indent, till it's flat over the indent, then allow to dry. I then polished over the keys and they are completly smooth again. It was also a good match to the original colour.