Why do the application instructions call for removal of excess material?

A properly executed application will leave no Ashford Formula residue on top of the treated surface. Only the material that has penetrated should remain. If un-reacted/un-penetrated material is left on the surface, it can create white patches if it comes in contact with water. If sufficient quantities of Ashford are left on the surface, then it can even turn white on its own, whether it comes in contact with water or not.