Fumed Colloidal Silica (also known as CAB-0-SIL)
For use when an extremely hard solid with high density is needed i.e. structural bonding filling and filleting. A small amount of Colloidal Silica ( about a tablespoon per 4 ounces) will add strength to a top coat but still finish absolutely clear.  The white color in the mixing cup will disappear when it cures.