Click “Show details” link in the effect to see what it does.
For example, for Flood filling: “Fill connected voxels with similar intensity.
Click in the image to add voxels that have similar intensity to the clicked voxel.” Therefore, it does not require any previous paints, you just click at any point in the image and similar-intensity regions are filled. If nothing is filled then it means you need to increase intensity tolerance or decrease neighborhood size.
To fill a hollow segment, use “Islands” effect’s “Add selected island” method and click in side the hole in the segment.
To fill many small holes at once, use “Smoothing” effect’s “Closing” method.