The proper product for surface application is a color hardener, which contains color, sand and cement and is broadcast on the surface and worked in while the concrete is still wet. Color hardener will actually produce a stronger surface because of the additional cement and sand contained in the product.
Prepare the floor by sanding the existing finish, whether it be to deglaze a varnish or sealer finish, or to smooth the finish. Thoroughly clean away the dust after sanding using a vacuum cleaner or slightly damp mop. Allow a wet floor to dry completely.