As a fire-drake, Smaug is naturally resistant to heat. Which means effectively they covered him molasses. It doesn't necessarily imply other forms of durability. Indeed, there's a line in the book about how falling in the lake would "quench" him, suggesting he's got a weakness to things that are anathema to fire. That last part isn't likely to matter against Kong, I'm just saying that Smaug has a distinct nature that should be taken into account before reading too much into the molten gold thing.