My goal was to see if water could be made to appear as smooth as glass by using a very long shutter speed - my goal was 20 seconds.
How is it possible to accomplish this?
First, a ND 2.1 filter was used - this filter provided about 7 stops of light attenuation.
Then a CPL was used to add approximately 2 more stops of attenuation.
And this combination was enough to achieve a 20 second exposure.