Keg cleaning - stubborn bits

[Pyrexic]

I just got a couple new-to-me corny kegs and in the process of cleaning and rebuilding them with new o-rings. One came out looking like new, but the other has a last little bit of something that I can't seem to break loose with hot oxy clean, scrubbing and even tried 12% hydrogen peroxide and baking soda. Been at this for a couple of days and wanted to see if there were any suggestions for next steps. Before all the efforts, I soaked in oxy clean for 2 weeks.

Thanks in advance. Here are pics of what is left.

 

[BOB357]

If you haven't already, use your fingernail or a toothpick to check and make sure those are some sort of buildup and not depressions in the metal. If they are indeed buildup, make a paste with Barkeepers Friend and a bit of water. Lightly scrub the affected area and then let it sit and dry. Once dry, give it a good rinse, using a cloth or sponge if needed to remove the paste.

 

[Pyrexic]

Thank you @BOB357. The Barkeepers Friend did it.