Can Top-Stop Vertical be used on horizontal applications?

Yes, but a major and potentially serious issue is over-application of Top-Stop Vertical on horizontal applications.  Because of the thicker viscosity, over-application and pooling of Top-Stop Vertical (and the active ingredients) is likely and will result in non-homogenous depth of surface etch, potentially leading to a non-uniformed appearance.  For this reason, we do not recommend the application of Top-Stop Vertical for horizontal applications.