Iron in the FC4x using a iron at 165C. After the ski has cooled, very lightly scrape. You also may make 1 or 2 light passes with the medium bronze brush if needed. FC4x is hard.
Continue brushing with the white nylon brush, then a horsehair or boars hair brush. Finish with the blue polishing brush.
The snow will be slow, 'squeaky' snow.
It is important to use the very hard CH4x wax, ironed in, scraped and brushed out well. The CH4x is even more important to use than the fluoro.
A dedicated cold grind ski with numerous layers of CH4x ironed in is recommended.
Note: FC4x over CH4x ran excellent in the very cold conditions at the SISU marathon.