I am not sure I understand exactly but if you are in the daylight you can not use that slow of a shutterspeed.
Now to take photos of waterfalls and to get the silky water (which is what I think you are asking about). I use a ND8 filter and in the daylight with a F8 Aperture its hard to get over 1/4 second without blowing out the highlights.
I now have 2 ND 8's but have not got to try them yet.
For instance this is with the ND filter and 1/10 second exposure it needed more time but not possible with only 1 ND filter.