gold testing stone: sandpaper? neutralize in baking soda & water?

I am new to gold testing and bought a natural obsidian stone for scratch acid testing. I find it really time consuming to clean the scratches off the stones. What do you use? Following the youtube videos instructions, I have been using a paper towel with water. But someone suggested soap could also be used. And this GIA article recommend 320 grit sandpaper "https://www.gia.edu/bench-tip-use-the-touchstone-method-for-testing-purity-karat-gold":

"Remove the metal from the stone’s surface by placing it facedown on a piece of 320-grit abrasive paper on a flat surface. Apply moderate pressure and sand it in a circular motion until sample layers are no longer visible. Neutralize, rinse and blot dry before replacing in the kit."

Has anyone used sandpaper?

The above GIA article also recommends something I did not see on any gold testing youtube videos--perhaps I don't understand it. It seems to say after using the acid, put the touchstone in a mixture of baking soda and water before evaluating the results:

"Now choose the bottle labeled for testing 10k and lightly swipe the applicator across all the samples.
• After approximately 20 to 40 seconds, place the touchstone in a mixture of baking soda and water to neutralize the acid in one beaker, then rinse in water in the other beaker.
• Blot the touchstone with paper towels and observe. The 10k sample has dissolved...."

 

I simply use an acid which is meant for more k than the gold tested, and wipe the scratch off. You could also use platinum testing acid.

 

Scotchbrit for me

 

I use a ceramic "stone" and wash it off with good old ivory soap. Available everywhere in the USA. Works fine. Soap with not much in it but soap, and cheap as chips here in the USA. Works a treat on eyeglasses too.