From what I understand of the discussion on THR is that this only applies to people who modify the finish of a gun they intend to sell. I.E. a FFL (gun dealer) refinishes a remington 700 in camo, and then attempts to sell it. This action would require a manufacturing license.
Joe blow takes his remington 700 to a FFl (gun dealer) and has his finish redone in camo. This does NOT require a manufacturing license.
Guns and Ammo Magazine's Surplus Special ran a thing a while ago about how to buy, refinish and sell milsurps. How to either cold-blue touchup minor bits, or that you could make your own garage hot bluing tank for about $200.
Then there's people who will Duracoat a gun for a friend.
And LOTS of people, even FFLs, will reblue or park scuffed-up guns in their possession before putting them out to sell, because it's a little work for a significant sales value.
And how do they know "intend to sell"? What if you refinish a gun, and then later decide to sell it? Was that "intent to sell"?
I can think of lots of ways ordinary people could get hung by this one...
And I'm not sure whether it's attempt to get more funds via more taxes, or just further discouraging of keeping existing guns in operating condition by allowing them to be easily refinished...i.e. removal of guns from the public through rust and other forms of attrition.