What is the method of using sand to allow attachments to stick on veneer surface? Can it damage the porcelain veneer?

My doctor has put 2 Invisalign attachments on my front tooth which has porcelain veneer, the surface was so smooth that the attachments couldn't stick for more than a few hours, after returning to his office he put some sand on my tooth saying that this will allow the new attachment to stick better. Any idea of what that is exactly and if this will damage my veneer? Thanks !