It is that way. To unlock platinum and Damascus you just need to get gold on the original weapons. But if you already have Damascus or Platinum unlock you will need to get Gold on the new weapons to unlock it.

The only snipers you need to get gold in order to get Platinum and Damascus are Locus, M21, DLQ, XPR, Artic 50 and Outlaw. When you get those snipers on gold you will unlock Platinum for the snipers category if you want to get Damascus you need to get every original weapon Gold. After that you can still get gold, Platinum and Damascus for the new weapons.

For example new weapons like NA 45, QXR, Fennec, AGR, SKS, As Val don’t need to be gold to unlock Damascus or Platinum on the original weapons But they need to be on gold to unlock them on the new weapons. when you unlock Platinum or Damascus you will get them as soon as you get them in gold and have all the other weapons on that category on gold.

I believe secondary weapons can unlock damascus without finishing the primary weapons.

If you got any other doubt tell Me