Yep yep, the YouTube vid was actually one that I studied before trying out the “cleaning”.
As for prevention… what would be the best option? I thought that zip-bag would give a decent amount of “protection”, turns out it wasn’t like that.

PS: most of my coins (ie: Silver Pandas, APMEX sealed etc) are looking spot on. I’m assuming it might also have something to do with coin treatment, before it leaves the mint.