Shower Touch-up

Background: My shower enclosure was installed new 29 years ago. It works fine. There are two soap dishes/holders/shelves molded into it, one on each corner. On the one that has received the most use over the years the white coating has rubbed away in several places, none larger than 1/16" diameter. What appears to be metal is showing through at those places.
What suggestions do you guys have for something I can buy at HD or Lowe's to touch-up those bare spots so they don't spread or grow thinner?
Former slayer of dragons; practice now limited to sacred
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