Hi, sprinkle some baking soda over the burn mark and then use a cloth or toothbrush soaked in vinegar to scrub the stained gently in a circular motion.
Here's a post that might help you - https://homeguides.sfgate.com/remove-burn-marks-porcelain-sinks-26789.html##targetText=Sprinkle%20baking%20soda%20over%20the,baking%20soda%20and%20vinegar%20residue.
I have used the baking soda vinegar method and it worked. Also I have bleached out light burn marks with lemon slices and juice from it.
Hello, Sprinkle baking soda over the burn mark. Dampen a cloth or a soft-bristled toothbrush with white distilled vinegar. Scrub the stained area gently in a circular motion to remove the stain. Rinse the area with cold water to remove any baking soda and vinegar residue.