Can my players coat a +1 Plate Armor with Adamantine?

My Dwarf smith wants to merge both his Plate Armor +1 and his Adamantine Plate Armor. The idea is to melt Adamantine, obtain the raw material, and coat his +1 Plate with it, thus obtaining a +1 Adamantine Plate Armor.

Would a +1 Adamantine Plate Armor be imbalanced?

Players are currently level 12, and I was thinking of charging 1000g for the task, alongside the consumption of one of the armors. The player would not be able to melt the new armor again, so this would be the final version of the item. I’m aware this is homebrew territory (you can only coat weapons with Adamantine), and my main concern is whether this item is overpowered or not. I can adjust its cost, whether it requires attunement, and possibly other effects/penalties.