The 'sand' marks are from shaping the parts decades ago. Apply the HCl acid with a toothbrush, and you'll see some white foaming indicating that the part is being partially consumed by the acid. You can scrub the part with an acid-soaked Scotch Brite pad to see if any spots are removed that way.

Rewetting the acid a few times will continue the foaming, but you'll probably want to put a stop to the reaction after about 10 minutes by washing off with running water.

The acid will dissolve any metal pad like Brillo and probably make a big mess.