Perhaps the advice indicates that the brightness of the image may still be too bright. If that is the case, a ND filter should complete the task, especially as (7/6)^2 is only 1.4 and the increased IR load will be quite small.
Maybe consider a full-aperture filter over the objective? I have used aluminized mylar film with great success, admittedly with smaller apertures.
This reply was modified 1 year, 2 months ago by Dr Paul Leyland. Reason: Fix lypo