It cannot be used with the lens hood. In order to avoid the problem of vignetting, the outer diameter is larger than that of ordinary filters at the beginning of the design. 

For this series, front thread diameter is bigger than the back thread diameter.
for example:
Back thread diameter (attach to lens): 40.5mm
Front thread diameter (attach additional filter/other accessories): 43mm
Variable ND filter size other accessories size

37mm	40.5mm
40.5mm	43mm
43mm	46mm
46mm	49mm
49mm	52mm
52mm	55mm
55mm	58mm
58mm	62mm
62mm	67mm
67mm	72mm
72mm	77mm
77mm	82mm
82mm	86mm

 

"X" cross on images is a common issue with wide angle lenses and variable ND filters. Physics limit the capabilities of variable ND filters on wide angle lenses.
When you enter the edge of the max range,the "X" issue may appear on your image. Because you are using it beyond its capability according to the laws of optical physics,which will inevitably cause this phenomenon.
 

No, the lens cap can only be used with K&F Concept variable ND filters. 

Hi,the ND fader filter has no X cross on images and it has new Nano coating layer technology, while the variable stop ND filter don't.