Vinegar is a staple tool of the fossil people. Many Texas fossils are embedded in limestone matrix, the mild acid in cooking vinegar ( 9% )is good for loosening
that last lingering "haze" on the fossil.
Acid on flint is just a personal option. I'm a water & toothbrush only person BUT I have to admit it's tempting for finds along the Devils river ( W TX )... Due to local mineralization all the flint is coated with a stubborn dirt, not scale !
Comes off after long water soaking but acid speeds up the desired results.
Sortta like wiping some slopped mustard off a 1851 Colt