I've been experimenting with long-exposure shooting using various ways to reduce light coming in through the lens, primarily by mounting a circular polarizer and, if it's still too bright, then adding a ND8. To really push things, I've also toyed with a cheap linear polarizer on top of them. It's a fun technique, and I yearn for a ND400, but till then I'll play with this combination. I have a lot to learn in the shooting and PP, but it's fun. Here are some results, first with the kit lens, the 18-55 Al II, with a CPl and deep shade:
#1 #2 #3
Here are Sigma 10-20mm shots with a CPl and the ND8:
#4 #5
And the Sigma with a CPl, the ND, and the cheap linear polarizer (the LPl, being cheap, imparts a strange color cast, so I converted to monochrome. ):
#6 #7