If a shop sands that silicone treated car before a good strong detergent wash followed by a solvent wipedown, they're only grinding the stuff in... not off. Even a fresh coat of armorall on interior will be vaporizing the whole time, contaminating the exterior. The problem isn't one of adhesion as much as the fisheyes it causes. Then again, people didn't really rub silicone all over the outside of their cars...till now. It is the nemisis of a good shoot-we hate it. Leah, only a layer of good wax (Meguires High Tech Yellow #10) and quick removal of the water will prevent it, and once burned in, only a buffer and compound will remove it. Not something you can do many times and not usually a do it yourself job.