Among the interior-varnished old masterpieces, there are the Stradivaris. We guess that Stradivari, early in his time, didn't come to the idea of interior varnishing. But in his late years, as his craftsmanship and experience approached near perfection, Stradivari started varnishing the internal surface of his works.
Andrea Bang presumes that the effect of this had been well appreciated by players of those days. That is why more of the Stradivari's later works were interior-varnished. Among 7 Stradivaris, which Maestro Andrea actually examined by himself, there were only 3 with varnished interiors.
Andrea Bang sees it ideal when the exterior and interior layers of varnishing penetrate into the wood and eventually meet each other. He calls this state the "Breathable Condition" to create perfect sound.