פירוש על עבודה זרה 94:15

The Talmud raises a difficulty on R. Ami. In the mishnah, stones seem to be analogous with the house. Since when it comes to the house, the plastering was on the outside and yet it was prohibited, so too when it comes to the stones. Even though the plastering is on the outside, the stones should be prohibited.
The solution is that when it comes to the house, there is plastering on the inside too, between the bricks.
The problem with this solution is that the Mishnah might be dealing with a case where the house was first plastered to be a regular house, and then was again plastered for it to be an idolatrous shrine. Such a house would be prohibited even though the plastering was on the outside.